The beam of roof window handing machine bears dynamic loads in operation. The rigidity of beam is the main factor that affects accuracy and stability of the whole machine. The stress analysis of beam was carried on firstly, then the mathematical model of topology optimization based on variable density method was established in which an additional constraint condition was taken part in. The iterative formula was deduced taking advantage of optimality criteria method and Lagrange multiplier was calculated using volume constraint. A filtering function was imported in order to solve checkerboard and meshindependent. After the preparation of MATLAB program, 3D model was created using the optimized cross section shape. Besides simulation of static stiffness was carried on to obtain the deformation with ANAYS, the test of static deformation also was implemented. The results of simulation and test were highly compatible to prove the accuracy and efficiency of the optimization and simulation .The final results are applied to the roof window handing machine and provide helpful instruction for similar optimization design.