Mobile Ad hoc Network (MANET) has attracted lots of research efforts these years, mainly due to its appealing applications in no infrastructure situations such as battle fields and disaster recovery operations . In this paper, we explore wireless Ad Hoc network focusing on its Routing Protocol AODV, we in depth analyze its operation principles and the problems exist in AODV routing protocol. AODV routing protocol is vulnerable, it is always exposed to many attacks, such as DoS, black hole, etc.  Through exhaustive researching the behaviors of black hole attack in NS2 simulation platform, and we finally propose a new routing security strategy to defense the black hole attack in AODV. Simulation results show that the strategy we proposed can not only effectively defense the black hole nodes, but also has better routing performance for moderate or small scale Ad Hoc Networks.