Abstract: By analysis and study of ⁴He/²⁰Ne、⁴He/⁴⁰Ar、N₂/Ar、H₂/Ar and CO₂/Ar ratios monitored in Guang Hua hot water well and in Xiao Tangshan warm spring, this paper discribs the equilibrium priciple of gas absorption and deabsorption on fault medium, and reasonably describs the mechanism of the obviously anomlous variation of various gas ratios in groundwater before earthquakes by acoustic emission theory. The anomalous variation of the gas ratios, especially the noble gas isotopic ratios in groundwater have very good relation wity earthquakes.