The invention relates to a threading machine knife rest with a steel tube axial automatic positioning mechanism. The knife rest comprises a knife rest body (1), an automatic lifting positioning mechanism and a controller, and the automatic lifting positioning mechanism is arranged on the side face of the knife rest body (1) in a sliding mode side, and the automatic lifting positioning mechanism is provided with a two-axis displacement sensor used for measuring horizontal displacement and vertical displacement of the automatic lifting positioning mechanism; and the controller is connected with the automatic lifting positioning mechanism, the controller controls the automatic lifting positioning mechanism to horizontally move to a set position along the side face of the knife rest body, and then the controller controls the automatic lifting positioning mechanism to ascend and descend to a set height; and a steel tube (5) moves, after impacting the automatic lifting positioning mechanism, the steel tube (5) stops moving and undergo positioning operation, and the controller controls the automatic lifting positioning mechanism to be restored to an initial position. Compared with the prior art, the threading machine knife rest has the advantages of being simple in structure, diverse in function and convenient to produce and use.