In order to realize a multipurpose integrated optical device, a power splitter with tunable splitting ratio is designed based on the modulation function of graphene-silicon waveguide. In the splitter, graphene layers are embedded in the waveguide of one arm of the Mach-Zehnder(MZ) structure, and an extra phase difference is introduced by applying bias voltage to obtain output power modulation. The finite-difference time-domain (FDTD) method is used to calculate the properties of the graphene-silicon waveguide, and then the transmission performance of the splitter is simulated. Both theoretical and simulation results show that the designed splitter has good optical performance, and its splitting ratio can be tuned by bias voltage.