The Wenchuan Earthquake Fault Scientific Drilling project was established shortly after the Wenchuan Earthquake. Several on-site laboratories were built to perform the real time fluid analysis during drilling simultaneously. The concentrations of argon, methane, hydrogen, carbon dioxide, helium, nitrogen, oxygen and radon in drilling mud gas were determined during the entirely process of drilling. The setup for real time fluid analysis was stability for long time. The mud gas such as methane and radon yielded low concentrations above the Principal Slip Zone (PSZ), whereas yielded high concentrations under the PSZ. The real time fluid data might provide the real time information for identifying and validating of the PSZ in the deep fault zone. The gas concentration showed abnormal fluctuation during the Ms 4.0 earthquake on April 27, 2010. The abnormality occurred one hour before the earthquake, and ended half an hour after the earthquake. The real time fluid analysis during drilling might have captured the signal of the Ms 4.0 earthquake at a quarter past six on April 27, one strong and nearest aftershock.