The invention belongs to the technical field of organic polymer plane optical waveguide devices and their preparation and relates to a slab-loaded waveguide thermo-optic switch based on a graphene heating electrode and a preparation method thereof. The whole device herein is of MZ1 optical waveguide structure, composed of an input straight waveguide, a 3-db Y branch splitter, a device modulating region composed of first and second interference arms parallel to each other, a 3-dBY branch coupler and an output straight waveguide; from top to bottom, the device modulating region of the MZI optical waveguide structure is composed of a silicone substrate, a silica lower covering layer, an optical waveguide core layer, a graphene heating electrode, a slab-loaded waveguide, a polymer upper covering layer and a metal heating electrode .