In order to reduce the interference of the on-line calibration of the ultrasonic flowmeter by the on-site spoiler, the large eddy simulation method is used to study the influence of the installation position along the flow direction and circumferential direction on the measurement accuracy when there is a 90° elbow upstream of the ultrasonic flowmeter, so as to obtain the best installation position of the ultrasonic flowmeter. The results show that the elbow causes the downstream flow pattern distortion. The farther the ultrasonic flowmeter is from the elbow, the lower its sensitivity to the circumferential installation angle, and the maximum error of the correction coefficient decreases from 0.11 to 0.03; the correction coefficient fluctuates with the change of the circumferential installation angle, and the higher the flow velocity in the pipe, the smaller the amplitude of the correction coefficient fluctuation, and the maximum fluctuation amplitude is reduced by 20.5%. In addition, when the circumferential installation angle is 0° and 180°, the sensitivity of the flowmeter to the installation position is lower, the correction coefficient is closer to 1.0, and the maximum error is within 0.04. The results are beneficial to improve the on-line calibration accuracy of the ultrasonic flowmeter under complicated flow conditions caused by the on-site spoiler.