The plastic deformation of the bottom of the broach seat was the main reason of the spindle could not be loose.Aiming at this problem,a design scheme was proposed,using face locking device to replace the original broach base plate to realize torque transmission function.The effective of the locking device was proved by mechanical parts strength analysis and checking.Through practical application of the device for one year in field,not only the problem of the deformation of the broach seat is solved,but also the selflocking function is increased and the mechanical performance of the main shaft loose broach mechanism is improved.