With the rapid development of meteorological modernization, more and more ultrasonic anemometers have been put into the meteorological field. In order to ensure the preparation and reliability of wind speed measurement data, it is necessary to analyze the uncertainty of its measurement results. According to the test methods and calibration points in the national standard of ultrasonic anemometer, taking the WMT700 type ultrasonic anemometer calibrated by the Air Velocity Laboratory of the National Meteorological Station as an example, five calibration points of wind speed are repeatedly measured, and the uncertainty of type A is evaluated according to the evaluation and expression requirements of JJF1059.1-2012 measurement uncertainty. At the same time, the sources of B uncertainty in the measurement process are analyzed and B uncertainty is evaluated. Finally, the extended uncertainty is obtained by synthesis. This method has reference value for the reliability evaluation of the measurement results of the ultrasonic anemometer.