Abstract: In this paper the situation of the observation and research of underground water tide in China and the result of theoretical study of the tidal mechanism of water level of a confined aquifer are introduced briefly. The partial differential equations of the influence of bulk strain due to earth tide,barometric pressure and oceanic tide on the water head in confined aquifer are given. The relations between the influence and the parameters of the aquifer are discussed according to the simplified solutions of the equations. In addition,the tidal coefficients and the phase lag of each tidal component of bulk strain due to earth tide for the water level at thirty-three representative wells chosen from China are calculated repeatedly by the method of harmonic analysis. The barometric coefficients of water level and the phase lag are also calculated.