In order to solve the problems resulted from dynamic access control in a user hier-archy, a cryptographic key assignment scheme is proposed by Prof. Lin to promote the performingability and to simplify the procedure. However, it may cause the security in danger as the userchanges his secret key; besides, some secret keys may be disclosed due to the unsuitable selectionof the security classes’ identities. Through setting up a one-way hash function onto Lin’s scheme,the proposed modification can greatly improve the security of Lin’s scheme.