The invention discloses a railway flat car loaded bridge girder erection machine and a girder erection method thereof. The railway flat car loaded bridge girder erection machine comprises a main girder, wherein a plurality of supporting legs are arranged at the lower part of the main girder; a stiffening device is arranged at the upper part of the main girder; a hoisting device is further arranged at the lower part of the main girder; and a self-moving device is arranged between the hoisting device and the main girder. The bridge girder erection machine is loaded on a railway flat car and a bridge to be erected is transported by using the flat car; and the hoisting device is used for grasping the bridge and the bridge is transported to a part needing to erect at the front part through the self-moving device. The transportation speed is greatly improved; the existing equipment on a railway line is utilized so that a special transportation system is saved; and construction equipment is simple and can be used for punching hole to erect.