Abstract: The ultrasonic flow metering system has obvious superiority in span ratio, reliability, repeatability, metering accuracy, pressure loss and maintenance strength. It has been the preferred metering instrument in large-scale flow and high-pressure natural gas metering. However, this system′s accuracy is usually influenced by many factors. Based on working principle and field application of ultrasonic metering system, some influence factors in ultrasonic flow metering system′s accuracy are analyzed including the effect of control valve noise, dirt in the inner wall of instrument and upstream straight pipeline, components of natural gas and metering accuracy of online chromatograph, temperature transmitter and pressure transmitter. It is also pointed out that checking and maintaining the ultrasonic flowmeter, reducing the influence of process installation conditions and insuring accuracy for relative instruments should be done well in field application.