Disclosed is a bypass valve comprising a valve body. The valve body has a gas inlet and a gas outlet, wherein a check valve is provided at the air outlet. The check valve comprises a check valve body, a ventilation plate and a first sealing gasket. A first gas passage for gas delivering is provided within the check valve body; ventilation holes are provided on the ventilation plate; the ventilation holes are in communication with the gas outlet; the first sealing gasket is provided between the ventilation plate and the check valve body; and the first gas passage and the ventilation holes are in selective communication with each other via the first sealing gasket. The uni-directional check valve is provided in the gas air outlet in the bypass valve, causing the direction of the gas flow to be unique, overcoming the problem of imprecise concentration of the anaesthetic gas caused by the a gas backflow, and having the advantage of structural technology and better gas tightness.