示例：Unity4.x

using UnityEngine;
using System.Collections;

public class SceneManager : MonoBehaviour {

    public void LoadSceneAsyn(string sceneName)
    {
        StartCoroutine(LoadScene(sceneName));
    }

    IEnumerator LoadScene(string sceneName)
    {
        yield return null;
        //场景AssetBundle路径
        string path = string.Format("file://{0}/res/scenes/{1}.scene", Application.streamingAssetsPath, sceneName);
        WWW www = new WWW(path);
        while (!www.isDone)
            yield return www;

        if (!string.IsNullOrEmpty(www.error))
        {
            Debug.LogError("加载资源失败 " + path + "\n" + www.error);
        }
        else
        {
            AssetBundle ab = www.assetBundle;
            //注意：场景名是打资源包时的资源名称，不是文件名称。
            Application.LoadLevel(sceneName);
            //注意：必须等场景成功激活后再调用Unload，否则会出现材质和shader丢失。
            ab.Unload(false);//释放镜像资源
        }
    }

     // Unity 2017
     private IEnumerator LoadSceneAsync(string sceneName)
     {
        string url = string.Format("file://{0}/scenes/{1}.scene", Application.streamingAssetsPath, sceneName);

        WWW w = new WWW (url);
        while (!w.isDone)
	yield return w;

        if (string.IsNullOrEmpty(w.error)) 
        {
            AssetBundle bundle = w.assetBundle;//访问assetBundle时所加载的资源才会解析到内存中
            AsyncOperation asy = SceneManager.LoadSceneAsync (sceneName, LoadSceneMode.Single);
            while (!asy.isDone)
		yield return asy;
            Debug.Log("ActiveScene>>>"+SceneManager.GetActiveScene().name);
        }
        else
        {
            Debug.LogErrorFormat("Load scene failure. Error={0}. \nurl={1}", w.error, url);
        }
     }
}

示例：Unity 2019

using System.Collections;
using System.IO;
using UnityEngine;
using UnityEngine.Networking;
using UnityEngine.SceneManagement;

public class Test : MonoBehaviour
{
    void Start()
    {
        string url = string.Format("file://{0}/scene/{1}.scene", Application.streamingAssetsPath, "scene2");
        //同步加载 (WWW)
        StartCoroutine(LoadScene(url));
        //异步加载 (WWW)
        //StartCoroutine(LoadSceneAsync(url));
        //异步加载 (UnityWebRequest)
        //StartCoroutine(RequestSceneAsync(url));
    }

    IEnumerator LoadScene(string url)
    {
        yield return null;
        WWW www = new WWW(url);
        while (!www.isDone)
            yield return www;

        if (!string.IsNullOrEmpty(www.error))
        {
            Debug.LogErrorFormat("加载资源失败 {0}\n{1}", www.error, www.url);
            yield break;
        }

        AssetBundle ab = www.assetBundle;
        string sceneName = Path.GetFileNameWithoutExtension(url);
        SceneManager.LoadScene(sceneName);
        ab.Unload(false);//释放镜像资源
        Debug.Log("Unload the scene assetBundle");
    }

    IEnumerator LoadSceneAsync(string url)
    {
        yield return null;
        WWW www = new WWW(url);
        while (!www.isDone)
            yield return www;

        if (!string.IsNullOrEmpty(www.error))
        {
            Debug.LogErrorFormat("加载资源失败 {0}\n{1}", www.error, www.url);
            yield break;
        }

        AssetBundle ab = www.assetBundle;
        string sceneName = Path.GetFileNameWithoutExtension(url);
        AsyncOperation async = SceneManager.LoadSceneAsync(sceneName);
        async.allowSceneActivation = true;
        async.completed += OnLoadSceneAsyncCompleted;
        //注意：下面这段代码会导致永远无法往下执行
        //while (!async.isDone)
            //yield return async;
        ab.Unload(false);//释放镜像资源
        Debug.Log("Unload the scene assetBundle");
    }

    private void OnLoadSceneAsyncCompleted(AsyncOperation async)
    {
        Debug.Log("OnLoadSceneAsyncCompleted()");
        async.completed -= OnLoadSceneAsyncCompleted;
    }

    IEnumerator RequestSceneAsync(string url)
    {
        yield return null;
        UnityWebRequest request = UnityWebRequestAssetBundle.GetAssetBundle(url);
        UnityWebRequestAsyncOperation req_async = request.SendWebRequest();
        while (!req_async.isDone)
            yield return req_async;

        AssetBundle assetBundle = DownloadHandlerAssetBundle.GetContent(request);
        string sceneName = Path.GetFileNameWithoutExtension(url);
        AsyncOperation async = SceneManager.LoadSceneAsync(sceneName, LoadSceneMode.Single);
        async.allowSceneActivation = true;
        async.completed += OnRequestSceneAsyncCompleted;
        //注意：下面这段代码会导致永远无法往下执行
        //while (!async.isDone)
            //yield return async;
        assetBundle.Unload(false);//释放镜像资源
        Debug.Log("Unload the scene assetBundle");
    }

    private void OnRequestSceneAsyncCompleted(AsyncOperation async)
    {
        Debug.LogFormat("OnRequestSceneAsyncCompleted isDone={0}", async.isDone);
        async.completed -= OnRequestSceneAsyncCompleted;
        if (async is UnityWebRequestAsyncOperation)
        {
            var asyncRequest = async as UnityWebRequestAsyncOperation;
            UnityWebRequest request = asyncRequest.webRequest;
            //TODO::
        }
    }
}