A subdomain hybrid cellular automata method for solving car body thickness optimization includes an outer loop and an inner loop: the outer loop is to conduct crash finite element simulation analysis, calculate an output response, and update a cell internal energy density, and update a target mass using a penalty function method; the inner loop is mainly to adjust a cell thickness using a PID control strategy according to internal energy densities of a current cell and neighboring cells thereof, so that a current mass of the inner loop converges to the target mass; and finally the cell internal energy density distribution approaches a step target internal energy density function as much as possible. Step target internal energy density update rules are provided in the inner loop. Cell thickness update rules based on a PID control strategy are provided in the inner loop.