Abstract: Biochar can be widely produced by heating the rice straw under an oxygen-deficient condition at 500 ℃, leading to loose porosity, high specific surface area, and strong adsorption properties. Fortunately, compacted biochar with amended clay has been proposed as a sustainable alternative material for the final cover of landfills in recent years. The pore structure of soil can also be changed (such as the pore size and porosity), particularly for the variation in the permeability of the overlying soil on the landfill site. The permeability coefficient of soil can be dominated by both the biochar content and the dry density. It is a high demand to accurately determine the permeability coefficient of biochar-clay mixed soil, in order to fully meet the functional requirements of the overlying layer of the landfill. This study aims to explore the influence of biochar content and dry density on the permeability of biochar clay mixed soil. Five types of biochar-clay mixed soil were selected with the mass percentage of biochar of 0%, 5%, 10%, 15%, and 20%. The dry densities were taken as 1.42, 1.56, and 1.65 g/cm³, which were 85%, 95%, and 100% of the maximum dry density of clay. According to the Technical code for municipal solid waste sanitary landfill closure (GB 51220 2017), the degree of compaction of the landfill cover soil was not less than 90% of the standard proctor maximum dry density (SPMDD) of clay. The compactness of the slope was not less than 85%, according to the national standard (GB 51220 2017). The self-developed device was also used for the flexible wall water-air joint penetration to measure the saturated permeability coefficient and air permeability coefficient of biochar-clay mixed soil with different biochar content and dry density. The relationship was obtained between biochar content, dry density, saturated permeability coefficient and gas permeability coefficient. The applicability and accuracy were verified by validation group tests. A biochar content-dry density-dependent air-water permeability function was then established using the optimal relationships. The results show that the water permeability values of soil samples that were treated with 5%, 10%, 15% and 20% were 8.25×10⁻¹⁷, 8.89×10⁻¹⁷, 10.40×10⁻¹⁷, and 18.25×10⁻¹⁷ m², respectively, under the small dry density, compared with the pure clay. Among them, the permeability of soil samples mixed with biochar 20% increased by nearly an order of magnitude. The biochar content was obtained under the same initial dry density from the test, according to the relationship curve between the air permeability of biochar-clay mixed soil. The functional relationship between the dry density of the sample and the biochar content was termed as the water-gas permeability function of the soil sample mixed with biochar. Anyway, this function can be expected to rapidly and accurately determine the permeability coefficient of soil samples, particularly with considering the dry density and biochar content. The easily measured permeability coefficient was also taken as an independent variable. The validation group test showed that there was better consistence between the calculated and measured water permeability coefficient, indicating the better applicability of the improved function.