Abstract: The relationship between the hydrogeochemical changes involving SO₄⁼, Cl^-, Ca⁺⁺, and Mg⁺⁺ in groundwater and earthquakes indicates that the chemical composition of many wells had changed before both the Haicheng earthquake (M = 7.3) and the Tang-shan earthquake (M = 7.8). On this background the mechanism of such hydrogeochemical changes is explored.The external cause of the hydrochemical changes in groundwater may be traced mainly to the changes of temperature and pressure. In the Panjin region, Liaoning Province, the changes in strata stress and flow pressure before the two earthquakes mentioned above distinctly reflect an increment of pressure near the ground surface at a distance of several hundred kilometers from the epicenters.The variations of SO₄⁼ and Cl^- on the explosion experiments and the Mg-ion increase in the process of water-turbidity before shocks imply the influence of pressure on the hydrochemical changes. The hydrogeochemical changes before shocks seem to be mostly caused by the strain of stressed water-bearing horizon and its country rocks.