As the communication technology weighs heavily in the power system, so the security issues becomes major concerns. So far most security research has focused on the SCADA system. In this paper we consider the security problems in the network environment of the distribution automation system (DAS) which is much different from the SCADA system. First we analyze the types of cyber threats in many applications of the distribution system, and formulate the security goals. Then we propose the efficient security algorithms to achieve these goals. The algorithm avoids complex computation of any encryption algorithm, considering the resource-constraint network nodes. We also propose the efficient secret key distribution algorithm without resort to the public key encryption. Finally we demonstrate the feasibility of the proposed security protocol by experiment.