Abstract: A screw rotating and advancing device for an injection molding machine, in which a movable pressure plate is provided with a servomotor to drive a ball screw in association with a ball nut provided at a rear fixing plate, so that the movable pressure plate homogeneously drives a screw shaft passing through a front fixing plate to perform the operations of injection, holding pressure, and measuring back-pressure; and accomplish a close-loop control of the pressure of molding material by using a force sensor. The other side of the movable pressure plate is provided with a servomotor to rotate the screw shaft, so that the rotational speed of the screw shaft can be precisely controlled. Furthermore, the present invention includes a pre-load system including at least two pre-loaded hydraulic cylinders, at least tow pistons and a communicating tube to balance the pressure difference between the prepressurized hydraulic cylinders.