Piston as one of the most important components in a diesel engine, its thermal load always causes fatigue failure. We may design the structure of a piston on purpose and reduce its thermal load with the temperature field. This paper describes the fundamental of thermal analysis for the diesel engine piston. A calculation model of the transient gas temperature and the transient heat exchange coefficient is built based on AVL-boost software, through calculation gets the top of the piston and the piston with cooling water heat exchange coefficient and temperature distribution, calculates the temperature field of the piston with the finite element method and modifies the calculation model by repeatedly comparing the result with the measured temperature. It is found out that the temperatures of the piston top and the first circular groove are proper after calculating the temperature field.