Aiming at the relatively low efficiency of gas flowmeter calibration, the gas flow online calibration system was developed based on master meter method. Based on working principle,the gas flow model was established,the structure of the master meter calibration system was constructed, the gas steady control was completed by PLC. The online flow measurement device was calibrated in KingView environment according to the collected data from master meter and flow measured meter amended to the standard value on the temperature and pressure values. At the same time, the system had the functions of data acquisition and processing, automatic printing report and data storage, to realize automation, high precision, high efficiency of the calibration.The standard device meets the design requirements through the measurement error and uncertainty analysis.