In order to solve the boundary determination difficulty between single parts when calculating heat transfer in diesel engines, especially the temperature and heat transfer coefficient between the solid cylinder and fluid coolant, a fluid-solid coupled system including piston-liner-coolant-body was brought forward. Modern FEA software could solve the fluid-solid coupled heat transfer problem that transformed outer boundary condition to inner boundary condition, so that heat transfer simulation became more reasonable and simpler. As an example, the steady heat transfer of a 3-D coupled system in a turbocharged diesel engine was simulated by FEM software named ANSYS. Temperature and flow field pictures were shown. Comparison between simulation and measurement data of piston key points showed that fluid-solid coupled method could solve the steady heat transfer between piston-liner-coolant-body and simulation results were in good agreement with experimental data.