To accomplish laser-induced thermal loading simulation tests for pistons, the Gaussian beam was modulated into multi-circular beam with specific intensity distribution. A reverse method was proposed to design the intensity distribution for the laser-induced thermal loading based on finite element (FE) analysis. Firstly, the FE model is improved by alternating parameters of boundary conditions and thermal-physical properties of piston material in a reasonable range, therefore it can simulate the experimental results of the originally designed shaping lens. Secondly, with the validated FE model, the target temperature field is achieved by alternating the intensity field and loading conditions, and the optimal design for the beam shaping lens is subsequently obtained. This reverse design method bridges temperature field and intensity field. The results show that numerical simulation can reduce design cycle and design expense efficiently. Meanwhile, this process can be taken as a kind of virtual experiment which makes the thermal loading test more controllable and predictable.