Abstract: In this paper, 21 samples of Chang 8_1 low permeability reservoirs in Longdong area of Ordos basin were tested for mercury intrusion, the microscopic characteristics of reservoirs were studied from three aspects: capillary pressure curve shape, pore throat distribution and fractal structure of pore structure. According to the capillary pressure curve and mercury injection parameters, the reservoirs in the study area were divided into four reservoir types. The heterogeneity of the reservoir was studied according to the peak shape, peak width and peak height of the pore histogram. It is found that there is no negative correlation between fractal dimension and porosity and permeability of Chang 8_1 reservoir, which indicates that the microscopic heterogeneity and physical properties of low permeability reservoir are not a simple negative correlation.