Abstract: In order to reduce the interference of flow obstructions on ultrasonic flowmeter online calibration, a large eddy simulation is performed on the flow field in the ultrasonic flowmeter downstream of the butterfly valve. The effects of the opening degree of the butterfly valve, the flow direction, and the circumferential installation position on the velocity distribution on the sound channel are analyzed. The results indicate that the butterfly valve causes severe distortion of the velocity distribution in the downstream pipe. With the increase of the ultrasonic flowmeter flow direction installation distance, the interference effect of the flow pattern distortion gradually weakens. The sensitivity of the correction coefficient to the circumferential installation angle also gradually decreases, and the maximum difference decreases from 0.12 to 0.04. The circumferential installation position of the ultrasonic flowmeter has a significant impact on the velocity distribution on its sound channel, resulting in fluctuations in the correction coefficient with the change of the circumferential installation angle. Furthermore, the farther away from the butterfly valve, the smaller the fluctuation value of the correction coefficient, especially when the circumferential installation angle is 0° and 180°, and the error of the correction coefficient is less than 0.04. The degree of flow pattern distortion in the pipe decreases with the increase of the butterfly valve opening, which improves the uniformity of the velocity distribution on the sound channel of the ultrasonic flowmeter. The recommended installation position of the ultrasonic flowmeter under the condition of butterfly valve disturbance is obtained, which is helpful in improving the accuracy of the online calibration.