my solution is :
- duplicate the mesh A, called B : A is for rendering , B is for casting shadow
- assign a material to B that “one minus” linking to opacity channel
- uncheck "Cast Shadow " in detail panel of A
- check “Volumetric Translucent Shadow” in detail panel of B
- unucheck “Render in Main Pass” in detail panel of B
(writen an article with images “substrate translucent shadow,unreal substrate/strata translucent shadow,ue 半透明阴影不显示问题的hack办法 - 知乎”)