The invention discloses a girder falling jack external locking device. The girder falling jack external locking device comprises a girder falling jack, and is characterized in that a plurality of steel hoops (2) are stacked up on the outer periphery of an extending end of a piston (3) of the jack; each hoop (2) consists of two semi-ring-shaped hoop bodies; each two semi-ring-shaped hoop bodies encircle the outer periphery of the piston (3) of the jack; the inner diameter of each hoop (2) is equal to the sum of the diameter (d) of the piston of the jack and 6mm; the outer diameter of each hoop (2) is equal to the sum of the diameter (D) of an oil cylinder of the jack and 100mm; and the height of the hoops (2) which are stacked up is equal to the distance between an end surface of the extending end of the piston (3) of the jack and an end surface of a piston extending end of the oil cylinder (1) of the jack. The girder falling jack external locking device is simple in structure and convenient to construct; the jack can be locked externally for a long time; and a supporting effect of the jack can be stable and reliable during girder falling construction, so that safety on girder falling construction is guaranteed.