Abstract: In this paper, we propose an identity-based online/offline encryption based on Waters-IBE, because of the limited computation and storage power of sensor nodes.This algorithm splits the encryption process into two phases:the offline and online parts.In the offline stage, all heavy computation can be done by PKG or base station prior to knowing the message and the recipient, the online stage can be conducted efficiently with some light computation to encrypt a message once the message and the recipient's identity become available.We have proved this algorithm could reach CPA security under the full model.What's more, the simulation results indicate that our algorithm reduces the requirement of computation and decrease the computation time in sensor nodes, so it is more suitable for applications in wireless sensor network.