Ad hoc networks are confronted with security challenges due to the characteristics such as dynamic topology, limited processing power, lack of fixed infrastructure and so on. In this paper, we propose a Security Policy based on Micropayment (SPM) to enhance the security of Ad hoc networks and the cooperation of nodes. SPM adopts scrip which issued by neighboring nodes to stimulate nodes to cooperate with each other and employs reputation mechanism to exclude the malicious nodes from the networks. Simulation results indicate that compared with other security policies, SPM makes Ad hoc networks more flexible and effective against misbehaviors of nodes and greatly controls the activities of nodes through the perfect combination of reputation and scrip.