set it be
context3D.setBlendFactors(Context3DBlendFactor.ONE,Context3DBlendFactor.ONE_MINUS_SOURCE_ALPHA)
solved my problem . two difference
(1,1,1,0.5)(1,1,1,1)+(0,0,0,1)(0.5,0.5,0.5,0.5)=(1,1,1,0.5) correct
(1,1,1,0.5)(0.5,0.5,0.5,0.5)+(0,0,0,1)(0.5,0.5,0.5,0.5)=(0.5,0.5,0.5,0.75) not correct