Based on careful selection of underground fluid observation wells (springs), and using the monthly means and subordinate function method, the authors select the precursory anomalies information of underground water level before the moderately strong earthquakes in Hainan Province and its vicinity and analyze the characteristics of the precursory anomalies. Statistics show that the average time of medium-short term anomalies in subordinate function is approximately one year before the earthquake, which can be used as reference time index for mid and short term earthquake prediction in the area.