PMI(Privilege Management Infrastructure) is a solution of the privilege management problem in grid environment, and has the advantage of distribution, dynamic, vast, and fine-grained. The calling of AC (Attribute Certificates) from LDAP server is a fundamental and frequent process on account of ACs holding authorization information in PMI. In grid, LDAP server is designed as a grid service, when an AC is called, additional time is taken on transferring the information on network. So it brings a bottleneck to the system?s performance. To solve this problem, we develop an AC cache mechanism. We research and implement the cache mechanism, and discuss the effects of the hit rate on the performance. An improved update algorithm is used to guarantee the hit rate by defining and identifying "not-often-use ACs". The experiment results show that the response performance of the PMI is improved indeed.