Performance simulations of one diesel engine were conducted before and after power enhancement respectively, so the boundary conditions of temperature field simulation were obtained, then 3-D steady state thermal analysis of the cylinder head was conducted with FEA software. The effect on heat transfer between boiling water and cylinder head were taken into account. And the experiment of measuring cylinder head's steady state temperature was made on a one-cylinder diesel engine, the results of simulation and experiment were compared. The rule of setting fire-deck's boundary condition, the relation between steady state temperature and the duty, the difference of temperature field between one-cylinder engine and multi-cylinder engine were obtained respectively.