The invention relates to a diameter changing mechanism of a numerical control pressure spring machine, which comprises a servo motor, a ball screw rod, a diameter changing base, diameter changing base fixing screws, a nut slide block, a fixed support seat, a diameter changing base press plate, a knife rest, a spring pushing pin, a spring pushing pin seat, a front wall plate and front wall plate arc grooves. The invention simplifies the mechanical control by reinforcing the control of computer software and discards the traditional mechanical approaches of complicated gears, screw rods, cams, and the like; the servo motor controlled by a computer is directly installed on the front wall plate, and a rotating shaft of the servo motor drives the slide block by the ball screw rod with high precision; the slide block drives the spring pushing pin fixed on the slide block to move back and forth and push or depart from a spring steel wire so as to carry out a diameter-changing working procedure for manufacturing the spring. The two arc grooves are arranged on the front wall plate of the pressure spring machine and can contain the diameter changing base fixing screws to slide or lock in thearc grooves, thereby adjusting the function direction of the spring pushing pin on the spring. The upper part and the lower part of the front wall plate are respectively provided with a set of diameter changing mechanisms, and one of the diameter changing mechanisms is selected to carry out operation according to the manufacture requirement of the left rotation or the right rotation of the spring, thereby controlling the outer diameter of the spring.