The present invention is a force detecting controlling device for a screw of an injection molding machine, which utilities a load cell as a force sensor for injection, holding pressure and back pressure. The load cell arranged on a rear fixed plate directly detects a force applied onto a ball screw nut. Because the ball screw nut is fastened on the load cell, and the load cell is fixed on the rear fixed plate, the force on a ball screw is directly transmitted to the ball screw nut. Therefore, a total value of the force on the ball screw produced by back pressure and friction among mechanical parts are equal to a force value of the ball screw nut applying onto the load cell. The injection force detected via the load cell for an instant response reaches a close loop control and procedures of holding pressure and back pressure are specifically controlled as well.