The invention discloses an assembled steel truss bridge with a track cantilever and a construction method thereof. The assembled steel truss bridge with the track cantilever comprises a horizontal running mechanism and a vertical lifting mechanism; the horizontal running mechanism comprises a truss girder, main truss slide rails, vertical slide rails, front assembling bolt plates and rear assembling bolt plates; the vertical lifting mechanism comprises a lifting bearing platform, a bearing platform slide rail, a lifting slide way and an upper limiting piece; the main truss slide rails are horizontally arranged at the top of the truss girder so that other truss girders can slide along the truss girder in a reciprocating manner; the vertical slide rails are vertically arranged on the side surface of the truss girder and used for the lifting slide way to slide up and down; the front assembling bolt plates and the rear assembling bolt plates are correspondingly arranged at the front part and the rear part of the truss girder and used for connecting other adjacent truss girders; one end of the lifting bearing platform is fixed; the other end of the lifting bearing platform slides alongthe vertically arranged lifting slide way; the bearing platform slide rail is arranged at the top of the lifting bearing platform and butt-jointed with the main truss slide rails; and an upper limiting piece is arranged on the external side of the lifting slide way and used for locating the lifting bearing platform.