Speed reduction locking device for telescopic boom, control method and engineering machinery

The invention discloses a speed reduction locking device for a telescopic boom, a control method and engineering machinery, the speed reduction locking device comprises a locking shaft, a telescopic locking mechanism and a speed reduction stop assembly, the telescopic locking mechanism is arranged on one of an outer boom and an inner boom, and the speed reduction stop assembly is arranged on the other of the outer boom and the inner boom. The speed reduction stopping assembly is arranged on the outer arm frame and is in driving connection with the locking shaft so as to drive the locking shaft to move towards the other one of the outer arm frame and the inner arm frame, and the speed reduction stopping assembly comprises at least two first stopping pieces which are sequentially arranged on the other one of the outer arm frame and the inner arm frame at intervals in the telescopic direction of the inner arm frame; the inner arm frame is arranged on the outer arm frame and abuts against the locking shaft to con