Abstract: The air valve is an important device for the air intake and ventilation of water pipelines and more and more used as water hammer protection measures, it is of vital importance to the selection of the air valve. This paper deduces two mathematical models of hypothetical air valves for ideal gas and incompressible gas respectively based on the basic principle of thermodynamics and analyzes the error of the air valve exhaust performance parameters according to the AWWA M51 guideline. The results show that the ideal gas hypothesis model has higher accuracy while the incompressible gas model has higher accuracy in the case of small pressure difference. This model can be used under the condition of small pressure difference such as initial filling and venting. This paper also uses the CJ/T217-2013 FGP composite air valve exhaust performance parameters for flow coefficient analysis. The results show that the flow coefficient is not constant but varies with pressure difference and pipe diameter. The flow coefficient should be measured by experiment in the calculation of the hydraulic transition process. The result can provide a theoretical basis for the type selection of air valves and project′s safe operation.