Immediately before an earthquake the pressure water layer may produce momentary flow under the action of pulse-shaped earthquake force. In the light of the unsteady flow theory the authors proposed a mechanism to calculate the theoretical curves of the water level drop and the flowing rate of the underground water in the abnormal boreholes according to the radial symmetrical flow model so as to imitate the typical observational curve of the uprising of well water. The developing process of this mechanism requires the formation and vanishing of a funnelshaped distribution of ground water level. Therefore, it is necessary to make further calculations on the theoretical abnormal curves of vertical deformation on the ground surface, gravity, geoelectricity and geomagnetism. These phenomena are all related to the growth and decay of the funnel. Before a strong earthquake, lots of abnormal points of uprising of well water type are generated. At the same time, due to variations of the ground stress and deformation field, the balance state of ground water distribution is disturbed and subsurface fluid flows along faults and crevasses. Thus, geophysical effects more complex than radial symmetrical flow will be produced. This is a shallow layer mechanism of short-term precursors.