The Delay Tolerant Network (DTN), as an intermittently connected network, transfers messages through opportunistic encounters between nodes. It is difficult to intercept messages because of the multi-hop and random delivery method, which shows that DTN is very suitable for covert transmission of information. However, it also limits transmission efficiency. In order to develop transmission efficiency and to guarantee the security of covert transmission, a routing algorithm was proposed. Firstly, an efficient message forwarding strategy was designed to improve the delivery efficiency of messages by utilizing the static social characteristics and real-time encounters of nodes. Secondly, because massive copies of message lead to high probability of delivery but also reduce the security of covert transmission, the number of copies was dynamically set according to the topology of the network. Finally, the proposed algorithm was simulated. Compared with the classical DTN algorithm, the simulating results show that the proposed algorithm can improve the message delivery rate of DTN，reduce network overhead, and increase the security of messages.