This paper deals with the mechanism of earthquake precursors on the basis of the chemical composition of underground water by means of injecting water test at an oil field. The test was carried out in the way that we made 10 wells simutaneously impounded up to a depth of 800-1200m. The observed well selected for analysing chemical composition of water is a water-bearing bed with its depth of 50-300m. The water source has nothing to do with the water-bearing bed of the observed wells, The pressure on the water-bearing bed increases as the impounding goes down up to the depth of 800m. The force for preparing earthquakes from the lower strata acting on the water chemical composition has been simulated by means of transmission of the increased pressure.
The observed value indicates that the chemical composition of under ground water varies when the impounding pressure is transmitted up to upper water-bearing bed to cause stress-strain process. It is obvious that the variation amplitude is related to the well tectonic settings, and its geochemical surroundings, but scarcely to the distance of observed well and the testing water-injection well.