Recently, cloud computing rapidly expands as an alternative to conventional computing due to it can provide a flexible, dynamic and resilient infrastructure for both academic and business environments. In public cloud environment, the client moves its data to public cloud server (PCS) and can not control its remote data. Thus, information security is an important problem in public cloud storage, such as data confidentiality, integrity, and availability. In some cases, the client has no ability to check its remote data possession, such as the client is in prison because of comitting crime, on the ocean-going vessel, in the battlefield because of the war, {\em et al}. It has to delegate the remote data possession checking task to some proxy. In this paper, we study proxy provable data possession (PPDP). In public clouds, PPDP is a matter of crucial importance when the client can not perform the remote data possession checking. We study the PPDP system model, security model and design method. Based on the bilinear pairing technique, we design an efficient PPDP protocol. Through security analysis and performance analysis, our protocol is provable secure and efficient.