Abstract: For the low accuracy of the common ultrasonic method in measuring the flow velocity in the pipeline with a very small diameter (below 5 mm), a correlation function method, which can accurately measure the time delay and reduce the influence of noise on the measurement results, is proposed in this paper for improving the accuracy of measuring flow velocity. First, the COMSOL Multiphysics finite element simulation software is used to establish the pipeline transmission model, then the ultrasonic waveform data of forward and reverse flows are respectively introduced into Matlab, and the correlation calculation is performed by the interpolation method to obtain a more accurate time delay, and finally the flow velocity in the pipeline with very small diameter pipes is calculated. The results show that the correlation function method combined with data interpolation can obtain a more accurate time delay and has a certain ability to suppress noise; the more interpolation points, the higher the resolution, but the anti-interference ability would decrease for too dense interpolation.