The utility model discloses a steel roof truss girder positioning support system, which is characterized in that a positioning frame is mounted on a cross beam, an L-shaped rotating shaft and a straight shaft are in threaded connection below the cross beam, a chassis is arranged at the bottom of the straight shaft, upright posts are vertically inserted into the chassis, the upright posts and the straight shaft are arranged at intervals, and a horizontal section of the L-shaped rotating shaft is clamped into a spacing space between the straight shaft and the upright posts. The positioning frameplays a role in bearing and hoisting the steel roof trussed beam; a space formed by an L-shaped rotating shaft, a straight shaft and a stand column which are in threaded connection below the cross beam clasps the bridge crane beam; when the bridge crane beam slides, the steel roof trussed beam on the falling frame is driven to slide, in the sliding process, the steel roof trussed beam is synchronously lifted by the hand-drive block to ascend and descend to avoid obstacles, finally, the steel roof trussed beam is installed, the vertical ascending and descending amplitude is large, and the costfor purchasing and installing a large special crane is saved.