In order to study the fuel temperature during the continuous climbing process of a UAV, the simulation model of the UAV fuel system is built by using the Flowmaster software. Taking the parameters of environment temperature, initial fuel temperature, initial fuel quantity and fuel consumption rate as boundary conditions, the fuel temperature in the fuel tank during the flight is obtained by simulation. The feasibility of the simulation model is verified by comparing with the actual flight test data. Based on the simulation model, the influence of environment temperature, fuel consumption, fuel consumption rate, climbing speed and other factors on the fuel change law is further quantitatively studied. The research results can provide scientific basis for the flight test of the UAV and the design of ventilation and pressurization system.