The invention discloses a piston type nut locking device. The piston type nut locking device comprises a nut absorbing part, a piston part, a lengthening rod part, and a connector part used for beingconnected with an external electric / pneumatic tool. The piston part drives the nut absorbing part to move when moving. During mounting, the nut absorbing part attracts a hex nut to align a screw for penetrating. The piston part is in the original state at this moment. The hex nut is jacked onto the screw, the external electric / pneumatic tool is started to drive the piston type nut locking device to rotate. The hex nut is screwed into the screw, the top of the screw jacks upwards, and the piston part is in the compression state. When the hex nut is locked, a clutch of the external electric / pneumatic tool is separated and turned off, so that the piston type nut locking device is separated from the screw, and the piston part reverts to the original state. The piston type nut locking device has the advantages that the assembling time is shortened, assembling efficiency is improved, assembling difficulty is lowered, and the piston type nut locking device is suitable for mounting nuts in mounting environments with different mounting depths.