The over fluctuation of indication often occurs when calibrating evaporating sensors of automatic weather stations in Sichuan Province. The measured errors can meet the requirement of the calibration specification at one time and cannot the other time, which result in poor repeatability of calibration data. Regarding such situation, it is hard to judge the quantification index of the measuring system on the basis of the field trial use calibration method of automatic weather stations published by the Department of Observation and Telecommunication, CMA. In accordance with JJF 10012011 General Measuring Terminology and Definition and JJF 11262004 Specifications for Calibration of Ultrasonic Thickness Gauge, etc., this technical problem can be solved by specifying the indication repeatability of the measuring system. Based on a great number of experiments and work experiences, the repeatability index and calibration method of the evaporation indications are developed, which provides theoretical reference for making specifications for the onsite calibration for the evaporating measuring system of automatic weather stations.