Abstract: This paper deals with the in-site real-time fluid analysis in the drill hole WFSD-4S. The abnormal fluid during the earthquake was discussed after excluding the influence of chemical reaction affected by mud additives and anomaly characteristics caused by lithologic changes. The research shows that two strong earthquakes were accompanied by abnormal occurrence of Rn, Ar, N₂ and O₂ in WFSD-4S. What deserves attention is the daily average concentrations of radon were over 2.5 times higher than the background value. The mud gas yielded high concentrations above Principal Slip Zone (PSZ) in 931 m,which should appear in 1084m through the lithology investigation. It is inferred that the underground gas migration channel was opened and gas migrated upward because of the earthquake preparation process, which resulted in anticipating the response characteristics of fault gases.