The invention discloses a method for main arch closure of three-span hinged arch bridges by adoption of pre-descended and pre-deflected side-span steel trusses. The method includes the steps: pouring side piers of two side spans, reserving post-pouring sections at the top of the side piers, and arranging temporary piers between each side pier and a main pier; enabling one side span steel truss to deflect towards a midspan direction by a pre-deviation distance in initial erection, enabling the two side span steel trusses to descend at the positions of the side piers by a pre-descending distance, and when the steel trusses are erected to the main pier, fixing fixed hinge supports on one side while temporarily fixing movable hinge supports on the other side, and adopting a first jack to jack the steel trusses to separate from the temporary piers; hoisting and erecting the steel trusses to a main arch closure position, and enabling the first jack to jack the steel trusses so as to realize main arch closure; unfixing the movable hinge supports, enabling the first jack to jack the steel trusses to a standard height, pouring the side piers, and freeing the first jack. The method has the advantages that safety and precision in main arch closure are achieved without adjustment of cables or monolithic movement of main truss arches, and accordingly construction risks are reduced while construction period is shortened.