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Presented a study on shale gas reservoir of Longmaxi Formation of Lower Silurian in southern Sichuan Basin. The reservoir porosity was measured using nitrogen adsorption method. The pore volume, pore specific surface area and pore size distribution were studied through the isotherms and DFF analysis. Based on all of these, discussed the main factors to control the nanopore structure and the nanopore's influence on shale gas accumulation. The results show that, the pore structure of the shale gas reservoir of Longmaxi Formation is relatively complex, which is composed of nanopores and possesses a certain ruleless structure (the amorphous pore), pores are open in style and mainly consist of the cylinder pores with two ends open and parallel-plate pores with four sides open. It is concluded that openness degree of the pores improved with the increase of the vertical depth, the sizes of the main nanopores are in a range of 2 ~40 nm, which take up 88.39% of the total volume of pores and 98.85% of specific surface area. Most of the pore volume is occupied by the mesopores which are 2 ~ 50 nm in size. The micropores and mesopores of which the diameters are less than 50 nm dominate the main pore specific surface. The main internal factor to control the volume and the pore specific surface of nanopores of the shale gas reservoir in Longmaxi Formation is TOC content. It is also the important substance to provide the main storage space. The nanopores have a strong ability to absorb shale gas, the interior of which is structured filled with the shale gas, thus increase shale gas storage capacity and finally make the model characteristic complicated. The open nanopores can improve the shale gas desorbing efficiency and the permeability of the reservoirs, and then improve the shale gas production. [ABSTRACT FROM AUTHOR]