With the development and wide application of dual-fuel engine technology, the existing average value model of diesel engine can no longer well realise the state monitoring of dual-fuel engine under different fuel mode operation modes, and it can’t complete the dynamic simulation of the changes in the composition of the intake and exhaust gas. In this paper, the average value method is used to establish a marine methanol dual-fuel engine model in Matlab-Simulink platform; based on the calculation of hydrocarbon fuel combustion products, a method of simulating and calculating the flow rate of each component in the intake and exhaust gases of the dual-fuel engine in the average model is proposed, which solves the problem of the average model of the diesel engine not being able to satisfy the control system’s requirements for the simulation and calculation of the composition of gases and gases. The problem of simulation calculation is solved. After completing the development of the model, simulation experiments were carried out under different methanol substitution rates and EGR rates, and the feasibility of the model was verified according to the simulation results, which can be applied to the dynamic simulation of real-time control model for ships and the simulation of control system for dual-fuel engines.