A novel kind of flywheel energy storage system structure for vehicle is put forward and studied. Flywheel rotor dimension optimization is achieved adopting genetic algorithm on MATLAB program, optimized dimension is helpful for the realization of high energy storage efficiency. Based on certain uniform ring simplification of flywheel rotor shape and applying elastic mechanics solution, theoretical calculation of radial displacement and stress is accomplished. Solved radial displacement, radial stress and hoop stress can be applied in practice for further precise design in the view of mechanical strength.