Abstract: Event driven wireless sensor networks (WSNs) have the characteristics of traffic bursting which leads to the congestion in local area. Thus, the data transmission reliability is deeply affected. At the same time, due to the scarce resources like energy, computational capability and storage space as well as rapid change in wireless link characteristics such as signal strength, interference, and multi-path propagation, how to provide a reliable data transmission in WSNs is an important and challenging issue. The mechanism of congestion aware routing is combined with network coding in this paper. A method of region congestion detecting is proposed. Based on neighbor nodes congestion degree, a formulation to calculate the congestion of this local area is given, which can indicate the congestion degree of the local area earlier and more accurately. And an on-demand congestion aware routing protocol based on network coding is proposed. During the routing discovery procedure, whether a node is selected to deliver data is according to its area congestion degree. The number of coding packets is properly estimated according to the link reliability. Simulation results in NS2 show that the congestion is avoided appropriately and the data delivery rate is improved. By reducing the effect of link invalidation, data transmission reliability is improved in wireless sensor networks.