Thermal analysis of aircraft hydraulic systems is an important subject for system’s design. Heat generation and dissipation of aircraft hydraulic systems had been discussed in this article. Heat exchange was calculated based on heat transfer theory. A formula of temperature variation was derived based on the assumption that oil flow in the hydraulic system was one-dimension unsteady flow. Take the hydraulic system of a certain commercial aircraft for example, a model was established to simulate the temperature change of aircraft hydraulic systems based on AMESim. The simulation results are in consistent with test data.