The invention discloses a torque multiplier and belongs to the technical field of wrenches. The torque multiplier comprises an input transmission shaft, a mounting cover is arranged outside the input transmission shaft, a ratchet and a shaft snap spring, the ratchet is fixed through a limit pin, and one side of the transmission input shaft is provided with a reversing lever; the tail gear of the input transmission shaft is engaged with a first planet gear, which is fixed onto a first-stage planet carrier through a first-stage planet pin, and the tail gear of the first-stage planet carrier is engaged with a last-stage planet gear, which is fixed onto a last-stage planet carrier through a last-stage planet pin; a gear ring is arranged on the outer ring of the last-stage planet carrier, and rolling pins are arranged between the last-stage planet carrier and the gear ring; an external reducer gearbox is arranged under the mounting cover and above the last-stage planet carrier, the first-stage planet carrier and the gear ring, an inner snap spring is arranged between the mounting cover and the reducer gearbox, and another inner snap spring is arranged between the gear ring and the reducer gearbox. The torque multiplier can be used in cooperation with a manual torque wrench, thereby simplifying highly-difficult operations, achieving high-torque assembly of bolts and nuts and obtaining a high security coefficient.