The invention relates to the technical field of immersed tube tunnels, in particular to a large component supporting system conversion device and a conversion method thereof. The conversion method ofthe large component supporting system conversion device comprises the steps of (a) top reinforcement cage hoisting; (b) internal supporting frame integral transferring; (c) bottom reinforcement cage lifting and bottom supporting system removing; (d) reinforcement cage mounting and inner formwork pouring; and (e) top reinforcement cage resetting. According to the conversion method of the large component supporting system conversion device, by lifting a top reinforcement cage, so that an internal supporting frame is integrally removed and integrally transferred, thus the problems of low efficiency, long duration and high cost in a conventional method of repeated disassembly, repeated assembly connection are solved, the construction efficiency is greatly improved, and the time cost and laborcost are greatly saved; and through the integral disassembly construction method, it can be ensured that the internal supporting frame keeps in a good integral shape, and the fact that reinforcement cage binding at each section has high individual difference is avoided.