With the low cost and low hardware complex considerations,cooperative systems are a tendency in the future communications.This work considers the secure cooperative communications systems.For a practical situation in the system,the scenario includes multiple source stations,multiple relay stations,multiple destination stations,and eavesdroppers.To analyze the optimal relay selection in the system,we begin with the performance analysis for a single source station and a single destination station.By applying two cooperative models,the amplify-andforward(AF) mode and decode-and-forward(DF)mode,the secrecy capacity is derived.Then,we apply the derived results to the considered environment to find the optimal relay assignment.By the way,the relay selection can be obtained by the exhaustive search algorithm.However,there are a lot of steps needed if the number of source stations is large.Hence,applying the characters of the cooperative modes in the relay selection,the pre-selection step is proposed with a mathematical derivation.It could be used for the practical situation without a long-time calculation. With the low cost and low hardware complex considerations, cooperative systems are a tendency in the future communications. This work considers the secure cooperative communications systems. For a practical situation in the system, the scenario includes multiple source stations, multiple relay stations, multiple destinations stations, and eavesdroppers. To analyze the optimal relay selection in the system, we begin with the performance analysis for a single source station and a single destination station. By applying two cooperative models, the amplify-and-forward (AF) mode and decode-and -forward (DF) mode, the secrecy capacity is derived.Then, we apply the derived results to the considered environment to find the optimal relay assignment.By the way, the relay selection can be obtained by the exhaustive search algorithm .However, there are a lot of steps needed if the number of source stations is large .ence, applying the characters of the cooperative modes in the relay selection, the pre-selection step is p roposed with a mathematical derivation. It could be used for the practical situation without a long-time calculation.