A power-assisted electric bicycle comprises a frame, a rear wheel and a driving unit, wherein, the frame includes a bottom bracket and a chain stay connected, with the rear wheel being rotatably mounted on the chain stay via a rear shaft; the driving unit includes a central shaft, a pedal chain wheel, two pedal devices, a freewheel and a chain; the central shaft is rotatably mounted in the bottom bracket, and the pedal chain wheel is mounted on the central shaft, with the two pedal devices mounted at both ends of the central shaft thereof, respectively; the freewheel is rotatably mounted on the rear shaft and cooperates with the rear wheel, so as to drive the rear wheel to rotate; the chain is connected with the pedal chain wheel and the freewheel, so as to transmit the driving force of the pedal chain wheel to the freewheel; wherein, the power-assisted electric bicycle further includes an electric power-assisted device which includes a torque sensing unit, an electric power-assisted unit and a control unit; the torque sensing unit is mounted in the bottom bracket and cooperates with the central shaft, so as to sense the torque generated on the central shaft by the pedal devices; the control unit is electrically connected with the torque sensing unit, so as to output a control signal to the electric power-assisted unit according to the information of torque sensed by the torque sensing unit; the electric power-assisted unit is mounted on the chain stay and cooperates with the chain, so as to output the corresponding driving force to the chain according to the control signal; the electric power-assisted unit includes a motor reducer, a drive chain wheel and a positioning mechanism; the drive chain wheel is mounted on the output shaft of the motor reducer, and cooperates with the drive chain wheel; the motor reducer is mounted on the chain stay via the positioning mechanism; wherein the chain stay includes a first beam and a second beam used for mounting mounted on of the rear wheel; the first beam and the second beam are mounted at their ends on the bottom bracket, with a roughly wedge-shaped mounting region formed closely to the bottom bracket at its narrow end, and away from that at its wider end; the motor reducer is mounted in the mounting region via the positioning mechanism; wherein the positioning mechanism includes a pressing plate and a first locking bolt nut, the pressing plate and the motor reducer being arranged above and below the mounting region respectively; and the pressing plate is detachably connected with the motor reducer via the first locking bolt nut; and characterised in that the pressing plate has a concave structure on its bottom for embedment of the mounting region, so as to prevent the pressing plate from shifting in the mounting region from the narrow end to the wider end; wherein the positioning mechanism includes at least one positioning block and at least one second locking member; with which the positioning block is detachably fixed via the at least one second locking bolt nut between the pressing plate and the motor reducer and between the first beam and the second beam, and tightly against inside of the first beam and the second beam, so as to prevent the pressing plate and the motor reducer from shifting in the mounting region from the wider end to the narrow end; wherein the at least one positioning block is detachably fixed on the motor reducer via the second locking bolt nut, and a through hole is formed on at least one positioning block allowing the second locking bolt nut to go through, with the through hole in a waist shaped so as to make the position of the positioning block relative to that of the motor reducer adjustable; wherein a chute is formed on the bottom of at least one positioning block, and a positioning raised-deck cooperating with the chute is formed on the top of the motor reducer; the distance between the centerline of the positioning raised-deck and the drive chain wheel is equal to that between the centerline of the bottom bracket and that of the plane of the pedal chain wheel.