The invention discloses a graphene-based waveguide integrated multimode electro-optic modulator and a manufacturing method thereof. The modulator comprises a first graphene nanoribbon layer, a secondgraphene nanoribbon layer, an insulator cladding, a multimode ridge waveguide, a first electrode pair, a second electrode pair, a third electrode pair, an insulating layer and a substrate layer, wherein the multimode ridge waveguide supports two transverse electric modes (TE), that is, a TE0 mode and a TE1 mode are simultaneously transmitted; the first graphene nanoribbon layer and the second graphene nanoribbon layer are integrated above the multimode ridge waveguide; the insulator cladding is used to achieve electrical isolation of the multimode ridge waveguide from the first graphene nanoribbon layer and the second graphene nanoribbon layer; the first electrode pair is connected to the first graphene nanoribbon layer, the second electrode pair is connected to the second graphene nanoribbon layer, and the third electrode pair is coupled to the multimode ridge waveguide for providing a back gate voltage to the graphene.