The invention relates to an electromagnetically-controlled three-gear automatic shifting speed reduction box for an electric vehicle. A working principle is in a way that power switching is achieved by electromagnet control and by combining calculation of a rotational speed, a torque and an accelerator signal, a first gear comprises an electromagnetic driver, a locking mechanism, a sun gear, a motor rotor, a planet carrier, a motor stator, a planet wheel, a gear ring and a motor output shaft, a second gear comprises the sun gear, the motor rotor, the gear ring, the planet carrier, the motor stator, the planet wheel, the motor output shaft, the electromagnetic driver and the locking mechanism, and a third gear comprises the planet carrier, the motor stator, the coil, the motor rotor, the sun gear, the motor output shaft, the planet wheel, the electromagnetic driver and the locking mechanism. The electromagnetically-controlled three-gear automatic shifting speed reduction box for the electric vehicle employs electromagnetic control and multi-gear selection, the requirements of maximum gradability and highest speed of the vehicle can be met, the electromagnetically-controlled three-gear automatic shifting speed reduction box is simple in structure and small in power loss and is difficult to damage, energy is saved, and uninterruptible power, no-impact flexibility and automatic shifting are achieved.