thanks for ink‘s help, I know the solution of my problem mentioned before.
New problem!!!!The Character became thin.It indeedly became thin.(It seems the all the shaders for dynamic font will make the printed character thin)
Please help me!Any help will be appreciated^_^
I think shade code has some problem.
If use this shader, font's edge is black.
In dark background, text is thin. In light background, text seems strange.
I fix it as follows. But i don't know this is right.
tex2D(_MainTex, IN.texcoord).a * IN.color;
-->
half4 col = IN.color;
col.a *= tex2D(_MainTex, IN.texcoord).a;