This paper systematically analyzes the data of borehole strain at Liu'an Seismic Station, Anhui. By analyzing the daily variation of the data, it is found that the daily variations can record the significant solid tides. But the form of long - period data feature is the " linear ascending-sudden dropping-linear ascending”. Because the drift rate of bore-hole strain is changing slowly, this paper uses the month as the window length to fit the strain data by a straight line and calculate the residual. This method can effectively eliminate the drift of borehole strain. The tides are calculated with Vendikov harmonic method using an hour-long window. Using the amplitude factor and phase lag of M2 wave as the reference for the abnormal data, magnitude 4.0 earthquakes occurred in Anhui province is analyzed. The results showed no significant abnormalities during the five months before the earthquakes.