Abstract: The breathing system of oil tank is a kind of safety system used to ensure safe operation of tank and reduce the evaporation loss of oil. Regular inspection and maintenance to the breathing system are important to ensure safe operation of oil tank. Some breathing systems used in above - ground tank and semi-underground tank, cavern tank, horizontal tank and heavy oil tank are respectively introduced. The inspection and maintenance methods of breathing system obout mechanical breather valve, hydraulic valve, and flame arrester used in ordinary tanks and the breathing system of cavern tank and heavy oil tank are described.