A breathing system for an anesthesia machine is provided, which comprises an operation mode switch, a CO2 canister, a bellows, a patient inspiratory branch and a patient expiratory branch. The operation mode switch comprises a vent communicating with the patient inspiratory branch, a vent communicating with the patient expiratory branch, a vent communicating with the bellows, and a vent communicating with the CO2 canister. The CO2 canister is positioned below the operation mode switch, so as to collect the water flowing through the operation mode switch.