The exploration of shale gas has been shown to be successful in North America and China. However，some controversy on the pore space and its formation still surrounded it. It is still not clear that whether the shale contain effective pores and how these pores influence the occurrence states of shale gas. To answer these questions，based on research achievements of preceding scholars and our experiments data of Longmaxi shale in Sichuan Basin，study is carried out on shale microscopic analysis of different areas，different rock types and different thermal maturities in this article. As a result，firstly，shale is classified in to organic-rich shale and silty-clay shale. Secondly，to organic-rich shale，the development of nanopore in organic matter （OM），the principal pore as well as the assemble and absorbed space of gas，is controlled by TOC content，especially the initial organic carbon content，so recovering the initial organic carbon is the critical factor for evaluating the gas resources in the shale. Thirdly，in low-maturity phase （Ro < 0.6%），there is no shale gas because OM pores is absent. With the shale developing to maturity phase （Ro = 0.8%），OM’s nanopores are abundant and shale gas is absorbed in them. As the maturity growing up to Ro > 1.2%，the micropore conver to mesoporous and macroporous pores，the specific surface areas decrease and the absorbed gas lessen，free gas content gets to increase.