Can render to screen, but can't render to renderTexture

hi everyone, I want to render something to a renderTexture by using functions of GL class, and then use this texture for render other things. I can get the correct result in rendering to screen, but can't render to the renderTexture. How can I do it? The script attached to mainCamera as follow, which with a diffuse material and a renderTexture draged to it. (Forgive my poor English, and Thanks in advance :-) )

 using UnityEngine;
 using System.Collections;
 
 public class RenderSomething2Texture : MonoBehaviour {
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
     
     }
     
     public Material mat;
     public RenderTexture _renderTexure;
     public GameObject cube;
     
     //Failed to render to textue.
     void OnPreRender(){
         cube.GetComponent<MeshRenderer>().material.mainTexture = _renderTexure;
         
         RenderTexture.active = _renderTexure;
         
         float offset = 0;
         
         GL.PushMatrix();
         GL.LoadOrtho();
     
         //GL.LoadPixelMatrix(0, (int) RenderTextureSetting.kBumpTexSize, 0, (int) RenderTextureSetting.kBumpTexSize);
         //GL.Viewport(new Rect(0, 0, (float) RenderTextureSetting.kBumpTexSize, (float) RenderTextureSetting.kBumpTexSize));
         GL.Clear( false, true, Color.black );
         
         mat.SetPass(0);
         
         
         GL.Begin(GL.TRIANGLE_STRIP);
         GL.TexCoord2(offset, 1.0f + offset);
         GL.Vertex3(0.0f, 1.0f, 0.5f);
         
         GL.TexCoord2(1.0f + offset, 1.0f + offset);
         GL.Vertex3(1.0f, 1.0f, 0.5f);
         
         GL.TexCoord2(offset, offset);
         GL.Vertex3(0.0f, 0.0f, 0.5f);
         
         GL.TexCoord2(1.0f + offset, offset);
         GL.Vertex3(1.0f, 0.0f, 0.5f);
         
         GL.End();
         
         GL.PopMatrix();     
         RenderTexture.active = null;
     }    
 
     
     //Render to screen is OK.
     void OnPostRender(){
         float offset = 0;
         
         GL.PushMatrix();
         GL.LoadOrtho();
     
         //GL.LoadPixelMatrix(0, (int) RenderTextureSetting.kBumpTexSize, 0, (int) RenderTextureSetting.kBumpTexSize);
         //GL.Viewport(new Rect(0, 0, (float) RenderTextureSetting.kBumpTexSize, (float) RenderTextureSetting.kBumpTexSize));
         GL.Clear( false, true, Color.black );
         
         mat.SetPass(0);
         
         
         GL.Begin(GL.TRIANGLE_STRIP);
         GL.TexCoord2(offset, 1.0f + offset);
         GL.Vertex3(0.0f, 1.0f, 0.5f);
         
         GL.TexCoord2(1.0f + offset, 1.0f + offset);
         GL.Vertex3(1.0f, 1.0f, 0.5f);
         
         GL.TexCoord2(offset, offset);
         GL.Vertex3(0.0f, 0.0f, 0.5f);
         
         GL.TexCoord2(1.0f + offset, offset);
         GL.Vertex3(1.0f, 0.0f, 0.5f);
         
         GL.End();
         
         GL.PopMatrix();     
     }        
 
 }