The invention provides an injection drive structure of an infection molding machine, wherein a filling screw of a feeding mechanism smoothly stirs plastic raw materials to fill the materials in a mould; when a filling hole is filled with the plastic raw material, a reacting force pushing the filling screw reversely can be generated so that the filling screw and a sliding mechanism assembled and connected with the filling screw are pushed backward; then an injection mechanism is driven by the backward sliding mechanism to actuate a drive screw of the injection mechanism and drive the sliding mechanism which is screwed with the drive screw, to push the filling screw to go forward and perform extrusion, injection and forming. According to the structure, the plastic raw material is smoothly stirred to be filled in the mould and the stepped movements of extrusion and injection are performed, thus the actuation load of the motor can be effectively reduced and the excellent rate of the injected and formed finished product can be increased.