Abstract: his paper classifies the acoustic emissions of sandstone into five stages according to the different deformation stages. They are the preliminary stage of loading, the density stage, the crack stable growth stage, the crack unsteady growth stage and the damage stage. According to distinct acoustic emission of each stage, it is found that the Kaiser point is relevant with the starting point of crack stable growth stage. The paper further presents that the upper limit of Kaiser points is the end-point of crack stable growth stage, or is the initial point of crack unsteady growth stage. The Kaiser point is affected by many factors such as parallelism between two planes of rock sample and loading rate of press machine. If the planes of rock sample are not flat, strong acoustic emission may occur at preliminary stage of loading. If loading rate is bigger, there may not be acoustic emission or the acoustic emission is covered up by latter signals. The paper calculates the values and directions of the three principal stresses of ground stress at 670 m depth below. Compared with field stress-relief method and hydraulic fracturing technique, the three principal values were basically consistent, but their directions were not wholly equal.