Abstract: As an effective method for coal reservoir characterization, mercury intrusion has been widely used in reservoir property analysis of medium-high rank coal. However, for low-rank coal, due to its loose and brittle nature, the process of mercury intrusion will destroy the pore structure, resulting in poor applicability of mercury intrusion experiments. In order to accurately assess the damage to the pore structure of low-rank coal by mercury intrusion experiments, lignite and long flame coal were selected as the research objects, and the pore size distribution of coal was tested by mercury intrusion and nuclear magnetic resonance. The results show that for lignite and long-flame coal with a low degree of compaction and gelification, the mercury intrusion test process destroys the primary structure of the macro-mesopores, resulting in an increase in the proportion of mesopores. At the same time, the micropore volume is too high due to the matrix compression effect. With the increase of coalification, the influence on long-flame coal is less than that on lignite. Moreover, nitrogen adsorption results were used to correct the test results of mercury injection under high pressure, eliminating the influence of elastic deformation caused by high pressure. The results show that the test error of lignite mercury intrusion before calibration is 87%, while after calibration it is only 18%. Experimental studies have shown that the combined use of nuclear magnetic resonance testing and nitrogen adsorption experiments can significantly improve the accuracy of mercury intrusion method used in low-rank coal pore test results.