UMTS is the most popular third-generation mobile phone system at present, and the UMTS AKA is his authentication and key agreement protocol. There are some weaknesses in the existed UMTS AKA protocol, such as synchronization question, storage overhead, insecurity with leakiness authentication vector, and weakness in mutual authentications. In this paper, we use the concept of proxy key to design a more secure and more efficient UMTS AKA protocol. Our protocol not only resists above-mentioned drawback of the original UMTS AKA protocol on the security, but also can improve the performance of UMTS AKA protocol, including the computational overhead, storage overhead, and communication overhead.