The great success of P2P (peer-to-peer) system based on Internet makes researchers focus on the mobile network environment, which is more distributed, with wider participants and more autonomic than the fixed P2P system. The popularity of intelligent terminals and the maturity of mobile application environment bring a bright prospect for mobile P2P networks. But the current research on the mobile P2P network is short of an accurate definition and a great number of questions are still needed to be studied deeply. In this paper, the basic concept of mobile P2P network is introduced firstly, including the definition and characteristics of mobile P2P network, the differences between mobile P2P network and mobile Ad Hoc network and the key techniques of the former. Then a comprehensive survey of the key techniques of mobile P2P networks, such as mobile P2P network architecture, resource discovery strategy, network structure consistency, data dissemination strategy, security and privacy mechanism, is given. The research results of the key techniques are also analyzed in depth, furthermore, the shortcomings and problems are outlined. In the end, the future research and development trend of mobile P2P networks is discussed.