Abstract: A shale gas reservoir is typically a continuous hydrocarbon reservoir, and its formation and enrichment cannot be separated from the shale. The generation, migration and accumulation of natural gas are the key factors for the effective accumulation of shale gas in shale. The Longmaxi shale in Sichuan Basin has excellent geochemical parameters and reservoir characteristics as indicated by organic geochemical analysis, X-ray diffraction and scanning electron microscopy and on drilling and outcrop data. We studied the tectonic evolution of well PY1 in Sichuan Basin and the thermal evolution history of organic matter in the Longmaxi shale. Compared to the Barnett shale, the Longmaxi shale was buried deeper, and was preserved in an open state due to tectonic activity, which was unfavorable for free gas accumulation. Kerogen in the Longmaxi shale belongs to types I and Ⅱ. The early formed hydrocarbons discharged out of the shale in the form of oil, and only the late formed, overmature gas and secondary gas (from oil cracking) remained in the shale system, restricting the effective accumulation of shale gas to some extent.