Alpha blend at RenderTexture

When I rendering 3D scene to a Texture, I got a incorrect result. I think the reason incorrect blend alpha is because the destination's alpha was ignored  on rendering. For example

1. make new RectangleTexture named A;
2. context3D.setRenderTexture(A); and context3D.setBlendFactors(SOURCE_ALPHA, ONE_MINUS_SOURCE_ALPHA);
3. context3D.clear(0,0,0,0);    In Result the A's alpha is 0
4. drawColor(1,0,0,1);            In Result the A's alpha is 1, and a red color;
5. drawColor(0,1,0,0.5);        In Result the A's alpha is 0.75  and color is (0.5, 0.5, 0, 0.75);

The result Alpha is 0.75, Not 1!

So, when rendering  A to the scene, It could't masked the background...

How to solve this problem, Who can help me, please.