A composite waveguide modulator based on graphene is proposed and the rigid three-dimensional numerical simulation results demonstrate the working states of this modulator. The surface plasmon polaritons in the slits of this modulator are used to strongly confine the electromagnetic wave, which enhances the interaction between the light and the graphene and thus improves the modulation performance of this modulator. Its modulation efficiency can reach 0.197 dB·μm -1. The on-off keying modulation with a modulation depth of 3 dB can be achieved when the modulator length is 15 μm. Meanwhile, the insertion loss is 0.4 dB and the modulation bandwidth is 1.85 GHz. The coupler connecting the input waveguide is designed, and the coupling efficiency can reach 86.6% when the coupler length is just 1 μm.