The invention discloses a variable-amplitude type girder erection crane and belongs to the field of cranes. According to the technical scheme, the crane comprises a crane bottom assembly, wherein a rear anchor piece and a jacking assembly located on a fulcrum are arranged on the lower portion of the crane bottom assembly, the rear anchor piece and the jacking assembly are used for supporting the crane bottom assembly, and a winch platform is arranged on the upper portion of the crane bottom assembly; a propeller strut which is fixed at the upper part of the crane bottom assembly; and an arm rod, wherein the lower part of the arm rod is hinged with the crane bottom assembly, the upper part of the arm rod is connected with the hoisting platform through a steel wire rope wound on the amplitude variation assembly, the upper end of the arm rod is fixedly connected with a lifting appliance through a connecting shaft; and a walking assembly which is fixedly connected with the jacking assembly, and has a first state of being inserted into the bridge deck clamping groove to be fixed to the bridge deck and a second state of being separated from the bridge deck clamping groove to move on thebridge deck. The problem of waste of manpower and time due to complicated operation is solved.