Abstract: In order to explore the measurement law of ultrasonic flowmeters under different rectifiers, a calibration pipeline model with rectifiers was established, the flow field of pipelines with different rectifiers was numerically simulated by Fluent software, and the indication errors of the ultrasonic flowmeters were obtained. Meanwhile, the accuracy of the numerical simulation method was verified based on the filed application data of flowmeters. The three types of rectifiers and ultrasonic flowmeters arranged on A, B and C channels were selected with the numerical simulation method, and the reasons for the measurement variations of ultrasonic flowmeters caused by the rectifiers were analyzed through the flow field distribution in the pipeline and the velocity distribution in the cross section at the flowmeter. The results show that different indication errors of different types of ultrasonic flowmeter are resulted from the three types of rectifiers. Specifically, the maximum and minimum differences of indication error for type A ultrasonic flowmeter are about 0.5% and 0.2%, that for type B ultrasonic flowmeter are about 0.1% and 0.05%, and that for type C ultrasonic flowmeter are about 0.4% and 0.2%, respectively. Base on that, suggestions are put forward for the installation and verification of ultrasonic flowmeters, which have guiding significance for improving the calibration quality of flowmeters.