The invention discloses a cover applied to a medical waste liquid collecting device and the medical waste liquid collecting device. The cover comprises a cover body part, a groove, through holes and a liquid inlet part, wherein the groove is formed in the surface of the cover body part in an inward concave mode, and the groove is provided with a groove wall; the through holes are formed in the groove wall of the groove; and the tail end of the liquid inlet part is arranged in the groove. The medical waste liquid collecting device comprises the cover and a main body part used for collecting medical waste liquid. The main body part is provided with an opening part. The cover is arranged at the opening part. According to the cover and the medical waste liquid collecting device disclosed by the invention, the liquid inlet part is connected with a return pipe of water jet scalpel type equipment and sucks cut tissues into the medical waste liquid collecting device, the tail end of the liquid inlet part is arranged in the groove, and when the return pipe is drawn out, medical waste liquid is effectively prevented from flowing out; and air in the medical waste liquid collecting device is exhausted through the through holes, and the medical waste liquid collecting device does not need to be connected with negative pressure.