Routing protocols in wireless sensor networks tend to be designed for a particular task type and network status. The dynamic routing system can adaptively choose the optimal performance of routing protocols at run time. Based on policy engine, a dynamic routing system is designed for wireless sensor networks. This work uses a modular design approach which makes each sensor node to carry multiple routing protocols. Resources, such as neighbor table and output buffer, are shared between routing protocols. A routing protocol switching mechanism is implemented based on the policy engine. Experimental results show that, in multitasking network environments, dynamic routing can meet the quality of service while effectively reducing energy consumption.