Abstract: Among the microscopic pore structure testing methods, nuclear magnetic resonance（NMR） testing technology has the advantages of wide testing pore range and fast testing speed. The mercury intrusion test technology can quantitatively study the soil pore distribution and pore size, but the test period is long. Therefore, a method based on mercury intrusion data to calibrate the NMR T₂ spectral curve was proposed, and this method was used to quantitatively study the pore structure characteristics of modified clay with different biochar contents and its influence mechanism on permeability characte-ristics. The results show that with the increase of biochar content, the saturated permeability coefficient of the biochar-clay mixture increases gradually. Adding biochar to clay changes the pore structure of the clay. For pure clay, the T₂ distribution curve has a unimodal structure. With the increase in biochar content, the T₂ distribution curve shows a bimodal structure, the T₂ spectrum shifts to the right, and the peak T₂ gradually increases. Combined with the proportional relationship between T₂ value and pore radius, it can be qualitatively judged that with the increase of biochar content, the most probable pore size of soil samples increases. Through the conversion coefficient C value between mercury intrusion data and NMR T₂ spectrum, the most probable pore size corresponding to peak T₂ is obtained, and the influence of the change of pore structure characteristics of biochar modified soil on the change of macroscopic permeability coefficient is quantitatively analyzed.