The invention belongs to the technical field of reactor internals of nuclear reactors and in particular relates to a metal reactor internal core vessel supporting structure of a high-temperature gas cooled reactor and a mounting method thereof. According to the technical scheme of the invention, the total weight (calculated by 800T) of the metal reactor internals, ceramic reactor internals and graphite nodules thereof is centralized on a circular supporting platform, and the newly designed circular supporting platform has the area of 9.684m<2> and the pressure of about 82.6T/m<2>, which are far lower than those of the original design. Six key grooves are formed in the supporting platform and a core vessel bottom plate, so that peripheral displacement of the core vessel can be effectively limited; two anti-trip bolt holes are respectively formed in two ends of each key groove, so that displacement of the core vessel in the upper and lower directions can be effectively prevented. Therefore, even if an earthquake occurs, the reactor internals do not have large play.