Abstract: Solar and wind power generation is with intermittent characteristic,which requires energy storage system to balance their effect on the power grid.The flywheel energy storage system（FESS） owns the advantages of quick response speed and high power density.So,the FESS can be used to solve the problem of grid connection for the large scale renewable power generation.The energy storage density is an important index to evaluate the performance of the FESS.In this paper,we proposed a method to increase the energy storage density of FESS.This method modifies the metal rotor structure to decrease the rotor mass and increase the energy storage density,while the proposed structure can still keep the original value of the total energy storage,rotation speed and rotor diameter.Firstly,the metal rotor with the same thickness is modified through a structure with different thickness in the radius direction.Then,the Response Surface Optimization Module in ANSYS Workbench is adopted to optimize the mass of the rotor structure above mentioned.Simulation results show that the developed structure can reduce 16.88% of the rotor mass and increase 20.32% of the energy storage density.