Abstract: The pore structure is the core content of shale reservoir evaluation,which has important influence on the reservoir performance,seepage capacity and shale gas productivity. In order to show the spatial characteristics of pore structure in shale reservoir comprehensively and visually,MATLAB programming was used to identify scanning electronic microscope(SEM)binary image of samples from Longmaxi shale in southern Sichuan Basin,by which different pore area and corresponding plane porosity were obtained,and the plane porosity function was fitted. Then,the conversion model between the 2D porosity function and the porosity function was established by applying the integral geometry theory. The function relationship between the porosity and the pore size was calculated and the shale pore structure of the Longmaxi Formation was analyzed quantitatively. The results show that the pore size ranges from 1 to 50 nm,13 nm corresponds to the MAX porosity,the volume fraction of mesopores is the most(93.7%). Compared with the results of high pressure mercury intrusion and low temperature nitrogen adsorption experiments,the result obtained by SEM image method is reliable,which verifies the feasibility of this method. The method is applicable for shale and tight reservoirs.