This paper analyses energy conversion and heat transfer in axial piston pump. According to it, the thermal mathematical model of axial piston pump is built by control volume method. This model can make full use of the dimension parameters of pump. The validity of the model is verified by the simulation for hydraulic system with axial piston pump. The result shows that this model can reflect and predict temperature changes of pump accurately based on leakage and friction. It also can give valuable reference for the axial piston pump design and fault diagnosis.