Objective To quantitatively compare the gas supply performance between balanced piston and balanced diaphragm first-stage pressure reducers,so as to provide reference data for establishing a diving apparatus evaluation system.Methods The gas supply performance test data of the type 15 first-stage pressure reducer under different input pressures were obtained through the unmanned performance test.Dynamic pressure drop,dynamic and static pressure difference,and maximum gas-supply flow rate were compared.Results There were statistical differences in dynamic pressure drop,dynamic and static pressure difference,and maximum gas supply flow rate between the 2 kinds of first-stage pressure reducers.The maximum supply flow rate linear regression model of balanced piston first-stage pressure reducers could explain 92.3%of the changes of the maximum supply flow.Conclusion The balanced piston first-stage pressure reducer is superior to the balanced diaphragm first-stage pressure reducer in dynamic pressure drop,dynamic and static pressure difference,and maximum air supply flow rate,but the internal isolation of the balanced diaphragm first-stage pressure reducer makes it more suitable for use in cold or dirty waters.