The invention discloses a graphene phase type light modulator based on a planar waveguide. The light modulator comprises a substrate layer, the light waveguide, a first dielectric filling layer, a second dielectric filling layer and an electrode structure. The first dielectric filling layer, the second dielectric filling layer and the light waveguide are located at the upper end of the substrate layer. The light waveguide is composed of a first rectangular waveguide body, a first isolation dielectric layer, a first graphene layer, a second isolation dielectric layer, a second graphene layer, a third isolation dielectric layer and a second rectangular waveguide body in sequence from bottom to top. The electrode structure comprises a first metal layer and a second metal layer, and the first metal layer and the second metal layer are deposited on the upper end face of the extending part of the first graphene layer and the upper end face of the extending part of the second graphene layer respectively. The graphene layers are embedded in the light waveguide bodies, interaction between graphene and light is enhanced, the resistance of a system is reduced, the modulation rate of the modulator is increased, the effective refractive index change in the waveguide can be effectively regulated, and dynamic modulation on a light field phase is achieved.