The utility model provides a multi -functional helping hand speed reducer, is mainly detected and is turned to by motor element, separation and reunion and step down gear mechanism, torque ratio and speed detection device, electrical control are constituteed. Characterized by: the separation and reunion comprises transmission shaft, center pin, gear, one -way freewheel clutch etc. With step down gear mechanism, and the speed reducing ratio is big, the torque ratio detects with turn to and the speed detection device mainly comprises half tensioning cover, terminal surface helical gear, hall element etc. Realizes the conversion of motion of gyration - straight line - sharp form, makes the corresponding displacement volume of comparison difference formation of pedal moment and running resistance square to the comparison that can ride moment and running resistance square simultaneously detects and turns to and the detection of rotational speed with pedal axle. Have pure foot tower drive function, pedal and motor helping hand hybrid -driven function, pure motor drive function, the integrated degree of mechanism is high, and output torque is big, thereby and pedal motion advantages such as driving chain mutually noninterfere are separated with electronic driving chain.