Security mechanisms for wireless sensor networks (WSN) face a great challenge due to the restriction of their small sizes and limited energy. Hence, many protocols for WSN are not designed with the consideration of security. Chaotic cryptosystems have the advantages of high security and little cost of time and space, so this paper proposes a secure cluster routing protocol based on chaotic encryption as well as a conventional symmetric encryption scheme. First, a principal-subordinate chaotic function called N-Logistic-tent is proposed. Data range is thus enlarged as compared to the basic Logistic map and the security is enhanced. In addition, the computation is easier, which does not take much resource. Then, a secure protocol is designed based on it. Most of communication data are encrypted by chaotic keys except the initialization by the base station. Analysis shows that the security of the protocol is improved with a low cost, and it has a balance between resource and security.