Abstract: The delay time △t of the change of water level compared to the change of earth tide at the wells of Yanzhou、NO. 49 of Henan. Tayuan of Bejing and NO. 14 of Shandon is respeetively about 18.2, 8.4, 9.2 and 22.0 minu-tes obtained by using the harmonic analysis method. It plays an important-role in the study of the relationship between underground water and stre-ss and strain of the earth's crust, and in the elimination of the influence of solid earth tide on water level.#br#The hourly values of water level in deep borehole at Yanzhou and Tay-uan wells during a period of nearly a month have been analysed by gradual regression method and the influence of various factors on the water level was eliminated. The obtained curve of residual △(t) shows random fluctuations. The residual standard error S is 6.8mm and 6.56mm respectively. The points out of the range of 2 S are only 2.6 to 3.4 per cent under the condition that the regional stress didn't produce the change of water level. The application of gradual regression method to the data processing of water level in a deep borehole, each independent variable and the relationship between indepondent variables have been discussed in detail, and the strict examination of precision and stabieity has been made.