A method of converting a bicycle into pedelec, wherein, it comprises the following steps: a bicycle is provided, comprising a bicycle frame, a rear wheel and a driving unit, wherein, the bicycle frame includes a bottom bracket and a chainstay connected, the rear wheel being rotatably mounted on the chainstay via a rear shaft; a 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 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; an electric power-assisted device is provided, which includes a torque sensing unit, an electric power-assisted unit and a control unit; the torque sensing unit is installed inside 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, according to the torque signal generated by the torque sensing unit, the control unit will transmit the control signal to electric power-assist unit; the electric power-assisted unit is mounted on the rear chainstay and cooperates with the chain according to the size of control signal, the electric power-assisted unit will indicate the corresponding driving force to the chain; the electric power-assisted unit includes a motor reducer, a driving chain wheel and a positioning mechanism; the drive chain wheel is mounted on the output shaft of the motor reducer, and cooperates with the driving chain wheel; the motor reducer is mounted on the chain stay by the positioning mechanism; wherein, the chainstay includes a first beam and a second beam that are mounted at their ends on the bottom bracket, where a wedge-shaped mounting area is formed near the bottom bracket, the narrow side of the mounting area is closed to the bottom bracket; the motor reducer is mounted on the rear chainstay by 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 area respectively; and the pressing plate is detachably connected with the motor reducer via the first locking bolt nut; characterised in that the pressing plate has a concave structure on its bottom for embedment suiting the mounting area, so as to prevent the pressing plate (623 8) from shifting from narrow to wider side; wherein, the positioning mechanism includes at least one piece positioning block and at least one piece second locking bolt nut; with which the positioning block is fixed by the at least one piece second locking bolt nut between the pressing plate and the motor reducer, then between the first beam and the second beam, tightly close the inside of the first beam and the second beam through positioning block, so as to prevent the pressing plate and the motor reducer from shifting from wider side to narrow side; wherein, at least one piece positioning block is fixed on the motor reducer by the second locking bolt nut, the mentioned positioning block is having a waisted-shape hole that allow the second locking bolt nut go through, so as to make the position of the positioning block and the motor reducer adjustable; wherein, at least one piece positioning block with a chute at the bottom, the top of the motor reducer formed a positioning raised-deck to suit the chute; wherein, the distance between the mid line of the positioning raised-deck to the driving chain wheel is equal to the distance between the mid line of the bottom bracket to mid line of the plane of pedal chain wheel. a bicycle is provided, comprising a bicycle frame, a rear wheel and a driving unit, wherein, the bicycle frame includes a bottom bracket and a chainstay connected, the rear wheel being rotatably mounted on the chainstay via a rear shaft; a 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 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; an electric power-assisted device is provided, which includes a torque sensing unit, an electric power-assisted unit and a control unit; the torque sensing unit is installed inside 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, according to the torque signal generated by the torque sensing unit, the control unit will transmit the control signal to electric power-assist unit; the electric power-assisted unit is mounted on the rear chainstay and cooperates with the chain according to the size of control signal, the electric power-assisted unit will indicate the corresponding driving force to the chain;