On the basis of the 9month observational data from five representative AWS in 2005, daily extremum distributions of main meteorological elements were obtained by the statistical method. Calibration period of various meteorological element s is studied by analyzing calibration time span and data transmitting schedule. The results show that the optimal calibration period of various meteorological e lements is as follows: humidity:8:00-11:00, fleet soil temperature:14:00-17:30, deepseated soil temperature:8:00-11:30, wind direction and wind speed:8:00 -9:3 0 or 16:30-18:00, barometric pressure:11:00-12:30,evaporation:8:00-8:30 or 17:30 -18:00. These results will help to revise the schedule of onthespot calibra tion of AWS, and reduce the influence of calibration operation on AWS observatio n.