参考：http://blog.csdn.net/bulademian/article/details/57082556

一、修改Sprites-Default.shader

Shader "Custom/Sprite-Outline"
{
	Properties
	{
		[PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
		_Color ("Tint", Color) = (1,1,1,1)
		[MaterialToggle] PixelSnap ("Pixel snap", Float) = 0
		
		// Add values to determine if outlining is enabled and outline color.  
        [MaterialToggle] Outline ("Outline", Float) = 0  
		_OutlineThickness ("Outline Thickness", Float) = 1.0
        _OutlineColor ("Outline Color", Color) = (0,1,0,1)
	}

	SubShader
	{
		Tags
		{ 
			"Queue"="Transparent" 
			"IgnoreProjector"="True" 
			"RenderType"="Transparent" 
			"PreviewType"="Plane"
			"CanUseSpriteAtlas"="True"
		}

		Cull Off
		Lighting Off
		ZWrite Off
		Blend One OneMinusSrcAlpha

		Pass
		{
		CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#pragma target 2.0
			#pragma multi_compile _ PIXELSNAP_ON
			#pragma multi_compile _ ETC1_EXTERNAL_ALPHA
			#include "UnityCG.cginc"
			
			struct appdata_t
			{
				float4 vertex   : POSITION;
				float4 color    : COLOR;
				float2 texcoord : TEXCOORD0;
				UNITY_VERTEX_INPUT_INSTANCE_ID
			};

			struct v2f
			{
				float4 vertex   : SV_POSITION;
				fixed4 color    : COLOR;
				float2 texcoord  : TEXCOORD0;
				UNITY_VERTEX_OUTPUT_STEREO
			};
			
			fixed4 _Color;
			float Outline;
			float _OutlineThickness;  
            fixed4 _OutlineColor;

			v2f vert(appdata_t IN)
			{
				v2f OUT;
				UNITY_SETUP_INSTANCE_ID(IN);
				UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(OUT);
				OUT.vertex = UnityObjectToClipPos(IN.vertex);
				OUT.texcoord = IN.texcoord;
				OUT.color = IN.color * _Color;
				#ifdef PIXELSNAP_ON
				OUT.vertex = UnityPixelSnap (OUT.vertex);
				#endif

				return OUT;
			}

			sampler2D _MainTex;
			float4 _MainTex_TexelSize;
			sampler2D _AlphaTex;

			fixed4 SampleSpriteTexture (float2 uv)
			{
				fixed4 color = tex2D (_MainTex, uv);

#if ETC1_EXTERNAL_ALPHA
				// get the color from an external texture (usecase: Alpha support for ETC1 on android)
				color.a = tex2D (_AlphaTex, uv).r;
#endif //ETC1_EXTERNAL_ALPHA

				return color;
			}

			fixed4 frag(v2f IN) : SV_Target
			{
				fixed4 c = SampleSpriteTexture (IN.texcoord) * IN.color;
				
				// If outline is enabled and there is a pixel, try to draw an outline.  
                if (Outline && c.a != 0) {  
                    // Get the neighbouring four pixels.  
                    fixed4 pixelUp = tex2D(_MainTex, IN.texcoord + fixed2(0, _MainTex_TexelSize.y*_OutlineThickness));  
                    fixed4 pixelDown = tex2D(_MainTex, IN.texcoord - fixed2(0, _MainTex_TexelSize.y*_OutlineThickness));  
                    fixed4 pixelRight = tex2D(_MainTex, IN.texcoord + fixed2(_MainTex_TexelSize.x*_OutlineThickness, 0));  
                    fixed4 pixelLeft = tex2D(_MainTex, IN.texcoord - fixed2(_MainTex_TexelSize.x*_OutlineThickness, 0));  
  
                    // If one of the neighbouring pixels is invisible, we render an outline.  
                    if (pixelUp.a * pixelDown.a * pixelRight.a * pixelLeft.a == 0) {  
                        c.rgba = fixed4(1, 1, 1, 1) * _OutlineColor;  
                    }  
                } 
				
				c.rgb *= c.a;
				return c;
			}
		ENDCG
		}
	}
}

Outline: 是否启用描边
Outline Thickness: 描边宽度(单位：像素)
Outline Color: 描边颜色

效果