Ultrasonic flowmeter has a good application prospect in the field of industrial production because of its good site applicability. In order to meet the requirements of the actual production for the accuracy of liquid flow measurement, the cross-correlation algorithm is used to detect the ultrasonic transit time (TOF), which reduces the calculation time error. Using FPGA internal resources to design and implement cross-correlation algorithm module, and adjust the logical structure to simplify the algorithm, reduce the resource duty cycle. The test results show that this method can effectively improve the measurement accuracy of transit time. In the range of 0-3000mm, the relative error of transit time is as low as 0.44%. The relative error of ultrasonic flowmeter is less than 3.68% in laminar flow region and less than 0.64% in turbulent flow region.