Abstract: Heavy metal contamination in soil has long been a focus of attention in the global environmental field. Heavy metal passivation remediation technology is a promising treatment technology. Biochar becomes a focus of research on heavy metal contaminated soil passivation remediation methods because of its green, efficient, and low-cost treatment characteristics. Furthermore, biochar demonstrated numerous benefits in terms of improving soil pH and organic carbon content, increasing soil water-holding capacity, lowering heavy metal active ingredients, and so on. To enhance the passivation remediation ability of biochar for Pb and Cd composite contaminated soil, rice straws were used as feedstock to prepare biochar (BC), and MNBC was obtained by BC modified with manganese dioxide and amino groups. The effect of the modification method and the addition amount of biochar on the basic physical and chemical properties of soil (pH, EC, organic matter, etc.) were studied. The changes in soil heavy metal availability and heavy metal chemical form (acid-soluble state, reducible state, oxidizable state, residue state) were analyzed. The results show that with the increase of the application of MNBC, the pH, EC, and organic matter of the soil increased. Manganese dioxide/amino modification significantly enhances the passivation performance of biochar. The application of 0.6% MNBC reduced 67.2% and 47.6% of the soil available Pb and Cd, respectively, i.e. 2.64- and 3.63-times that of BC. MNBC can effectively change the chemical forms of heavy metals by reducing 10.0% and 18.0% of soil available Pb and Cd, and increasing 12.6% and 9.1% of soil residual Pb and Cd, respectively. Overall, the addition of biochar significantly reduces the Pb and Cd in the soil and improved the physical and chemical properties of the soil. MNBC is an efficient material for passivation of Pb and Cd in soil.