Abstract: Geographical routing in wireless sensor networks is vulnerable to multiple security threats such as false routing information, selective forwarding and the Sybil. A trust based secure model is put forward for defending against above-mentioned attacks. For the nature of sensor nodes' resource constraint, trust values of neighboring nodes on the routing path can be calculated by Dirichlet distribution, which is based on data packets' acknowledges in a certain period instead of the energy-consuming monitoring. Trust is determined by combining the cost of geographic routing with energy aware routing for selecting the next hop of routing. Meanwhile, the initial trust is dynamically determined. In accordance with the trust value, service levels are divided to restrict malicious nodes' requirements. Simulation results and analysis show that, under multiple attacks, the proposed model has advantages in successful delivery ratio and network lifetime over the existing models.