found the error, because I changed the namespace and assembly name, then on the bin folder the old dll was still there, so it looks like the mvc engine searches for controllers in the entire bin folder。
也就是说报此错误的可能原因是bin目录下有就的dll产生了干扰。
http://stackoverflow.com/questions/23587380/multiple-types-were-found-that-match-the-controller-named-home-weird-error/23665265