A volume regulating structure for a rotary helical screw compressor includes a housing, a male-female rotor pair, a slide valve, and an extending support. The housing is provided, inside, with a rotor chamber and a slide valve chamber which are opening at the discharge surface of the housing. The extending support is fixed on the discharge surface of the housing, and that an inner profile of the extending support is aligned with two lower-half arcs of an opening profile of the slide valve chamber. Accordingly, part of the slide valve moved out of the housing can be supported by the extending support so as to reduce shocking of the slide valve effectively.