Could you describe what behaviour this is trying to achieve? C-w deletes the previous work in the search prompt in vim.

same to C-w in isearch, search words under point ,and you can press C-w multiple times

Yes, actually C-w should have the opposite behavior. It should delete the previous word in the Ex minibuffer since that is how vim does it. That is more in line with how evil is meant to behave anyways since its main goal is to emulate vi/vim.

But there are variables like evil-want-C-u-scroll which take into account discrepancies between fundamental keys used in both Emacs and Vim but have different meaning. Perhaps this could be another one of those cases and we could have an
evil-want-C-w-delete-in-search variable or just piggyback off of evil-want-C-w-delete? I'm thinking the latter makes more sense since if someone is already used to the Emacs way of C-w meaning to kill text, they wouldn't want that behavior to be different in Ex/search. If this is done then it would probably make sense for evil-ex-completion-map to also obey evil-want-C-w-delete.