The invention relates to an MZI type optical waveguide hybrid integrated thermo-optical switch with a graphene doped trapezoidal cladding, and a preparation method of the switch, and belongs to the technical field of optical waveguide type thermo-optical switch preparation. The method comprises the following steps: spin-coating a silicon dioxide substrate with a photosensitive polymer core layer material; performing photoetching on the photosensitive polymer core layer material by adopting a mask to obtain an optical waveguide core layer; then performing spin-coating of a graphene-doped polymer cladding material, evaporating of a metal film, spin-coating of a photoresist, registering and photoetching, and developing to prepare an electrode pattern; and finally performing cleavage so as toprepare the MZI type optical waveguide hybrid integrated thermo-optical switch with the graphene-doped trapezoidal cladding on the substrate. According to the method, the trapezoidal cladding structure is used for replacing a flat cladding structure, so that the heating efficiency of an electrode is improved; and the polymer material doped with single-layer graphene dispersion liquid is adopted toreplace a traditional polymer material, so that the problems of high power consumption and long switch time caused by low heat conductivity of polymer are solved. The method has the excellent effectsof simple process flow, high preparation precision and the like.