Abstract: After Qingyang-Xianyang Pipeline in Lanzhou-Zhengzhou-Changsha Products Pipeline was put into operation, the pipeline capacity in initial station and terminal station was calculated by means of readings measured from ultrasonic flowmeter. However, there was a greater difference from actual pipeline capacity for diesel and gasoline. In the light of this problem, it is found out that the main factor which affects measuring accuracy is Reynolds number. Analysis results show that ultrasonic flowmeter mainly works on the oil properties, temperature and flow rate. From the above, it can be known that theoretical measurement error range of diesel oil and gasoline is about 5 % to 6% and close to actual operation measurement error under the same measurement condition, thus theoretically explaining the reasons for pipeline measurement deviation. In accordance with measured data of ultrasonic flowmeters in the Qingyang-Xianyang Pipeline, solutions to measurement error are proposed to improve the accuracy of tracking/forecast for mixed oil interface.