The invention discloses a self-control suspension back pack based on motion perception and an acquisition method for optimal effective length. The device comprises a main body frame, a sliding module,a back pack rack, a pulley block, an elastic rope, a second detection unit, a clamping module, a drive module, a control module, a power supply, a first tension and pressure sensor and a second tension and pressure sensor, wherein the sliding module slides along the main body frame; the back pack rack is connected with the sliding module; two ends of the first tension and pressure sensor are connected with the sliding module and the elastic rope respectively; the elastic rope passes through the pulley block and the clamping module; the elastic rope is connected with the second tension and pressure sensor; the drive module is used for driving the clamping module; the power supply is used for supplying power for the drive module and the control module; the pulley block and the clamping module are connected with the main body frame respectively; and the control module is used for adjusting an optimal rope length so as to send a rotating command to the drive module. According to the method, stepwise regression is established in a cross multiplication form according to independent variable interaction and high-order influence, and the optimal effective length is acquired through variance analysis; and according to the method, the load impact can be alleviated.