Self-heating has become a limited factor for the performance improvement of GaN electronics. Growing polycrystalline diamond directly on GaN material to solve the heating problem of GaN devices has become one of the research highlights. Polycrystalline diamond on Si-based GaN material has the advantages of being close to the channel region and high heat dissipation efficiency. However, there is a problem that the thermal expansion mismatch between polycrystalline diamond and GaN material leads to the deterioration of electrical characteristics of GaN. In this work, we adopt microwave plasma chemical vapor deposition (MPCVD) method to grow polycrystalline diamond on 2-inch Si-based GaN material. The test results show that the polycrystalline diamond is uniform as a whole. The average thickness is in the range of 9–81 μm. With the thickness of polycrystalline diamond increasing, the XRD (002) diffraction peak FWHM increment and mobility loss gradually increase for the Si-based GaN material. Through laser cutting and acid etching, the Si-based GaN material is successfully stripped from the polycrystalline diamond. It is found that during the process of diamond growth at high temperature, hydrogen atoms etch the defect positions of the silicon nitride epitaxial layer, forming a hole area in the GaN, and the etching depth can reach the intrinsic GaN layer. During the process of cooling, a crack area is formed around the hole area. Raman characteristic peaks, full widths at half maximum of XRD (002) diffraction peaks, and electrical properties of the stripped Si-based GaN materials are all returned to their intrinsic states. The above results show that the thermal expansion mismatch between polycrystalline diamond and Si-based GaN introduces stress into GaN, which leads to lattice distortion of GaN lattice and the degradation of electrical property of GaN material. The degradation of GaN material is recoverable, but not destructive.