Due to the addition of ultrasonic reflection device, the U-shaped ultrasonic flowmeter has will disturb the flow in the pipeline. In order to explore the influence of the disturbed flow field on the measurement accuracy of the ultrasonic flowmeter and propose a feasible correction method, the disturbed flow field and the ultrasonic coupled field were simulated by using the software, Fluent and Comsol. The Reynolds number effect, the unsteady effect of the disturbed flow field and its influence on the ultrasonic propagation were discussed. Besides a feasible correction based on Reynolds number and Mach number was suggested.