This paper mainly focuses on the numerical simulation research of the time-difference method ultrasonic flowmeter. The time-difference method is used to measure the flow rate of fluid in the main flow pipeline, and the calculation time-difference and numerical simulation time-difference are compared under several conditions of mono-and cross-sound circuit respectively, which verifies the reliability of numerical simulation time-difference. The comparison between the time difference measured by mono and cross channels verifies that increasing the measurement sound path can improve the measurement accuracy. At the same time, the measurement accuracy of mono-path and cross-path under different conditions is compared, and it is found that the cross-path is more suitable for the measurement of fast flow rate, and its measurement accuracy increases with the increase of flow rate.