The invention discloses a method for erecting a steel girder of a cable-stayed bridge with diagonal main trusses. The method comprises the following steps that A10, a diagonal web member is mounted, lifting steel wire ropes are fixedly arranged on the diagonal web member, leading chains are mounted on the lifting steel wire ropes, the diagonal angle and the twist angle of the diagonal web member are regulated through the leading chains after the diagonal web member is lifted to a position to be erected by utilizing a girder erection crane, and then the mounting of the diagonal web member is completed; A20, lower chords, a lower layer bridge deck system and vertical web members are lifted and mounted by utilizing the girder erection crane; A30, a sway bracing is mounted in an auxiliary manner by utilizing a movable type vertical support frame; A40, upper chords are mounted by utilizing the girder erection crane; and A50, an upper layer bridge deck system is lifted and mounted at twice in the longitudinal direction of the bridge. The method has the advantages of simplicity in operation and short construction period, and the problems of difficulty in space positioning and mounting of the diagonal web member, high middle span deflection of the mounting state of the sway bracing and large welding workload of the upper layer bridge deck system are solved.