Abstract: In order to solve the problem of ultrasonic attenuation caused by the upstream elbow disturbing the flow field and the particle impurities in the sewage, simulation calculations were performed based on the Fluent Discrete Phase Model (DPM) bidirectional coupling model. When the pollutant concentration was 1%, four calcium carbonate impurities of different diameters were set. In the sewage with impurities of different diameters, the flow coefficient k value gradually decreased with the increase of the flow point, but at the small flow point of 20m3/h, the maximum influence of 80 large particle impurities on the coefficient k exceeded 5%, and the flow rate was not enough to keep the larger particles suspended in the fluid, resulting in uneven distribution of particle concentration in the flow field and large fluctuations in the speed of the ultrasonic wave on the propagation path. It was found in the pressure cloud map that with the increase of particle diameter, the fluid velocity pressure gradient on the pipe section changed greatly, resulting in violent fluctuations in the flow field, which in turn affected the change in speed on the ultrasonic wave propagation path and caused errors in the flowmeter. In order to reduce the turbulent pulsation caused by large particle impurities in the downstream, a rectifier was set at the bend, and the Laws rectifier physical model was established using Solidworks. The study found that the downstream flow field was more uniform and the velocity gradient changed less, reducing the influence of the backflow vortex on the ultrasonic wave on the transmission path. It has guiding significance for improving the uniformity of downstream flow field and improving the measurement accuracy of ultrasonic flowmeter in sewage.