Based on the analysis on the related characteristics of enormous ground temperature data at observatories in Jiangsu Province, the general law of the appearing time of the best index point is found. With singlestationed data, the statistical index and empirical index are established and the deviation between estimated value and correct value is compared and analyzed. Conclusion is drawn as follows: In most cases, the deviation between the estimated value and correct value, which are established by the two indexes, would come within a given range which is scientifically rational. This is the important basis for the daily malfunction judgment of ground temperature sensors.