Abstract: Machala coal strata is one of the main coal stratums in the coal-bearing belt of Changdu. In order to clarify the influence of weathering on the pore structure characteristics of Machala coal formation in Tibet,in this paper we took different types of weathered rocks from the Early Carboniferous Machala Formation section in Tibet as the research object,and investigated the microscopic pore structure and its complexity by using nitrogen adsorption test and fractal theory. The test results show that the sorption and desorption isotherms of the rocks show an inverse "S" shape and the pore structure shows a homogeneous trend. The relative pressure(P/P₀)can be divided into three stages: less than 0.1,0.1~0.45 and more than 0.45 according to the rising trend of the sorption and desorption curve. The pore structure of the rock samples is concentrated in the range of 1~5 nm,among which the pore volume of muddy clastic limestone,chalky mudstone,siltstone,mudstone and limestone is larger in the range of 1~2 nm,and the specific surface area and pore volume of the coal seam are lower compared with those of the adjacent rock layers. The fractal dimension of macropores is significantly higher than that of micropores,and the complexity of pore structure is higher. The fractal dimension of pores in coal seams and carbonaceous mudstone is lower compared with other types of rocks. The results of the study can be used for the development and disaster prevention in the Changdu area.