In order to solve the illegal collusion such as information concealment and false reporting in security assessment, the mechanism design theory was introduced to build a tripartite contract design model for government, port enterprises and assessment institutions under the condition of information asymmetry and in view of the uncertainty of security state type and security signal type, and the impact of security state types on contract parameters and the optimal policy choice of each subject were also analyzed. The results shows that under certain conditions, the government can effectively restrain collusion and improve the security state by adjusting the contract parameters such as information rent, throughput and port fees of port enterprises. Under the established supervision, there is a threshold for the security state, below the threshold, it is necessary to achieve the anti-collusion through the dual mechanism of “incentive + punishment”, and above the threshold, only the “punishment” mechanism is required to achieve anti-collusion.