Abstract: The gas well pressure data of Zhongba gas field in the vicinity of Longmenshan fault zone were analyzed in this paper and significant changes were found in most oil wells before the 12 May 2008 Wenchuan earthquake. The well pressures of 7 deep wells at Lei-3 gas storage layer with mining depth of 3100 m simultaneously rose with rates greater than 35%mdash;130% from April 8 to May 5 and recovered from May 6 to the occurrence of earthquake. At the same time, the pressures of most of 24 gas wells at Xu-2 gas storage layer with mining depth of 2400 m decreased slowly with the maximum rate of 45%. The pressures of a few wells rose before the earthquake, especially No.52 and No.63 wells showed 24% and 15% well pressure increase around May 5. Several days before the earthquake, the well pressures at the two gas storage layers basically returned to normal state. After the earthquake, changes of well pressure varied, but most of them recovered after increasing for 7mdash;8 days. The physical mechanism of abnormal change of wellhead pressures at the two gas storage layers with different mining depths before the earthquake is also discussed. Based on the dilatancy theory of earthquake precursors and gas dynamic process of hydrocarbon migration during the development of earthquake, this paper gave a possible explanation of the physical mechanism of abnormal well pressure changes.