According to the automatic soil moisture monitoring data read from the database of National Meteorological Information Centre (NMIC), the soil bulk density, field capacity and wilting moisture of each station are calculated. In the operational practice, according to the soil maximum hygroscopicity, the lowest threshold of the relative soil moisture is set to be 6%. According to the saturated soil moisture, the top threshold of the relative soil moisture is set to be 190%. According to the diurnal variation characteristics of the soil moisture, the diurnal variation amplitude of the relative soil moisture of 10 cm and 20 cm soil layer is set to 0.1%. The variation range of relative soil moisture with time of representative stations is analyzed. The results show that in the increasing stage of soil moisture, the variation amplitude between hours should be less than the difference of the saturated soil moisture (%) with the previous monitoring data. When the relative soil moisture is larger than 100%, the decline range should be less than the saturated soil moisture (%) minus 95%. And when the relative soil moisture is less than 100%, the decline range should be less than 5%.