Flow measurement is the most important factor affecting the test accuracy of hydro turbine efficiency. Ultrasonic method is mainly used in the flow measurement of large diameterpipe. However, there is no effective calibration method for its measurement accuracy and error composition. Based on the principle of ultrasonic time difference flow measurement, the comprehensive flow error is deduced and the flow measurement error description model is established. A quantization method for error analysis based on ideal flow measurement system is proposed, which provides an analytical approach for error analysis and control of ultrasonic flow measurement system. The influence of ideal flow measurement system on the accuracy of ultrasonic flow measurement is simulated. The influence of the measurement errors of various parameters on the comprehensive errors of the system is analyzed. The relevant correction methods are proposed for the major influential factors, and the control of the comprehensive error of the system is analyzed. Finally, the experimental system is set up, and the experimental results preliminarily verify the effectiveness of the proposed method.