The invention discloses a cantilever assembling construction method of a multi-main truss steel truss structure, which relates to the technical field of bridge construction. The method comprises the following steps: an N-th inter-section bridge deck is pre-installed between an N-th inter-section edge truss and an N-th inter-section center truss; the (N+1) th intersegment side truss is erected; theN-th intersegmental bridge deck is fixedly connected with the N-th intersegmental side truss and the N-th intersegmental center truss; the (N+1) th Intersegment Center Truss is erected; steps S1 to S4 are repeated to assemble the cantilever until the edge spans the pier and the middle span closure. The construction method of the invention can effectively control the edges of the multi-main trusssteel truss girder structure, the height difference of the middle truss is in a very small range, and the bridge deck alignment is ensured.