Aiming at the influence of upstream elbow pipe flow field changes on the measurement accuracy of ultrasonic flowmeter, CFD is used to numerical simulate the flow field inside the measuring pipe, and a rectifier is designed to improve the obvious secondary flow and eddy current caused by the bend, so as to reduce the measurement error of ultrasonic flowmeter. The research object is DN15 ultrasonic liquid flowmeter based on time difference method. The flow range is in the range of 0. 1~ 1. 5 m 3 / h. and the distance between the upstream elbow and the flowmeter is 2 ~ 20D. Compared with the measurement errors of ultrasonic flowmeter before and after installing the rectifier, The experimental results verify that the flowmeter has a greater measurement error with the shorter the straight pipe segment when the flowmeter is not rectified. The installed rectifier can improve the speed distribution of the flow field in the pipeline, shorten the length of the straight pipe section to 10D, improve the ultrasonic flowmeter measurement error to meet within ± 1. 5% , verify the correctness of the numerical simulation, and have certain guiding significance for the practical application of the project.