A torque sensor assembly for a power-assisted bicycle comprising a torque sensor having an envelope tube, a middle tube axially mounted through the envelope tube, a transmission axle axially mounted through the middle tube, at least one outer bearing mounted around and between the envelope tube and the middle tube, at least one inner bearing mounted around and between the middle tube and the transmission axle, a first disc, a second disc, and a detection unit detecting shift angle difference between the first disc and the second disc, the torque sensor assembly further comprising a one-way bearing securely mounted around one of two ends of the middle tube, a chainwheel securely mounted around the one-way bearing, two crank respectively connected securely to two ends of the transmission axle, and two pedals respectively connected rotatably to the cranks, and the torque sensor assembly characterized in that: the middle tube has a first stop (124, 124A) formed on an inner surface of the middle tube; and a second stop (125, 125A) formed on the inner surface of the middle tube and separated from the first stop (124, 124A) of the middle tube; the transmission axle has a first stop (134, 134A) formed on an outer surface of the transmission axle and disposed between the first and second stops (124, 125) of the middle tube; and a second stop (135, 135A) formed on the outer surface of the transmission axle, separated from the first stop (134, 134A) of the transmission axle, disposed between the first stop (134, 134A) of the transmission axle and the second stop (125, 125A) of the middle tube and selectively abutting the second stop (125, 125A) of the middle tube; the torque sensor further has a resilient element (16, 16A) disposed between the middle tube and the transmission axle and having two ends respectively connected to the middle tube and the transmission axle; the first disc being annular, and mounted around and attached to the transmission axle; the second disc being annular, mounted around the transmission axle, disposed adjacent to the first disc and attached to the middle tube; and the detection unit mounted on the envelope tube. the middle tube has a first stop (124, 124A) formed on an inner surface of the middle tube; and a second stop (125, 125A) formed on the inner surface of the middle tube and separated from the first stop (124, 124A) of the middle tube; a first stop (124, 124A) formed on an inner surface of the middle tube; and a second stop (125, 125A) formed on the inner surface of the middle tube and separated from the first stop (124, 124A) of the middle tube; the transmission axle has a first stop (134, 134A) formed on an outer surface of the transmission axle and disposed between the first and second stops (124, 125) of the middle tube; and a second stop (135, 135A) formed on the outer surface of the transmission axle, separated from the first stop (134, 134A) of the transmission axle, disposed between the first stop (134, 134A) of the transmission axle and the second stop (125, 125A) of the middle tube and selectively abutting the second stop (125, 125A) of the middle tube; a first stop (134, 134A) formed on an outer surface of the transmission axle and disposed between the first and second stops (124, 125) of the middle tube; and a second stop (135, 135A) formed on the outer surface of the transmission axle, separated from the first stop (134, 134A) of the transmission axle, disposed between the first stop (134, 134A) of the transmission axle and the second stop (125, 125A) of the middle tube and selectively abutting the second stop (125, 125A) of the middle tube; the torque sensor further has a resilient element (16, 16A) disposed between the middle tube and the transmission axle and having two ends respectively connected to the middle tube and the transmission axle; the first disc being annular, and mounted around and attached to the transmission axle; the second disc being annular, mounted around the transmission axle, disposed adjacent to the first disc and attached to the middle tube; and the detection unit mounted on the envelope tube.