The invention relates to a vacuum pump, in particular to a screw vacuum pump with the internal pressure ratio changeable, and belongs to the technical field of screw vacuum pumps. The screw vacuum pump comprises a pump body, a drive rotor and a driven rotor which are located in the pump body, a changeable protecting seat and a valve element, wherein a seal cavity is defined by spiral grooves in the drive rotor and the driven rotor and the inner wall of the pump body, the changeable protecting seat is arranged at the exhaust end of the seal cavity and provided with at least two exhaust holes, the valve element is rotatably installed in the changeable protecting seat and provided with a closing plate matched with the exhaust holes, and the outer side of the valve element is connected with a rotational driving device. According to the screw vacuum pump with the internal pressure ratio changeable, the size of the internal pressure ratio can be automatically adjusted according to the actual working condition, the using requirements of high vacuum degree can be met, and meanwhile the energy consumption can be significantly lowered.