To accomplish laser-induced thermal loading simulation tests for pistons, space intensity distribution of laser is changed by diffractive optical element (DOE),and time distribution of laser is controlled with the help of computer. It is necessary to know proportion of intensity distribution for design and process of DOE. Finite mode of intensity distribution is established. Results show temperature distributing of piston is agreement with target temperature distributing when power proportion from center circle to outer circle is 0.03,0.01 and 9.96. Thermal loading of high cycle and low cycle can be simulated by optimization DOE.