Based on a study of the shale gas reservoir of the Wufeng Formation-Longmaxi Formation of Upper Ordovician-Lower Silurian in the Qilongcun section, Xishui County on the south margin of Sichuan Basin, this paper analyzes the relationship between the pore size of the black shale and the occurrence of gas. The micropore characteristics are described by several methods, including quantitative analysis using nitrogen adsorption method, the qualitative analysis being scanned by scanning electron microscopy, and the ratio analysis of total organic carbon （TOC） and Si/A1. The characteristics of the pore structure of the shale are as follow. （1） The sizes of the main micropores of the samples with SiO2≤70% are in a range of 3--10 nm, taking up 50%-83% （The average is 71%） of the total volume of pores and 86--97% （The average is 94%.） of the specific surface area. （2） The obvious positive correlation between the methane adsorption capacities, the ratio of Si/ A1, TOC and the pore volume and specific surface area of the micropores （The pore diameter is 〈10 nm. ） is shown. These results imply that the micropores （The pore diameter is 〈10 nm. ） are the main spaces of reservoiring gas in the Wufeng Formation-Longmaxi Formation of Upper Ordovician- Lower Silurian on the south margin of Sichuan Basin.