The invention discloses a high-formwork multi-combined high load floor support system construction method comprising the steps that (a), cantilever floor plate construction is carried out; a track crane car is led below the cantilever floor plate, and the area covered by the track crane car is a structural steel support area; and the area with the minimum load on an upper layer floor of the structural steel support area is a trolley parking area; (b), a trussed beam is installed above a spent fuel vertical shaft area, the area covered by the trussed beam is the structural steel support area, and other parts are a full arch support area; (c), a structural steel support framed is built in the structural steel support area, and a full arch scaffold is built in the full arch support area; and(d), a trussed track and a hoisting trussed car are installed on the trussed beam. The high-formwork multi-combined high load floor support system construction method is used for solving the problem of the high-formwork and high load upper layer floor construction under complex working conditions after the introduction of large lifting equipment for a spent fuel plant of an nuclear power plant inthe existing technology, and the purpose of achieving stable overall support for the high-formwork and high load upper layer floor is achieved.