The study was aimed to solve the problems of relatively extensive cultivation and low soil quality of Pandanus amaryllifolius that restrict the improvement of yield and quality. Different ratios of biochar combined with mushroom residue were used to improve the soil, clarify its yield-increasing effect and mechanism, and to provide technical support for efficient cultivation of Pandanus. Under basic fertilization, four treatments were set up, 800 kg/hm² mushroom residue (MD), 800 kg/hm² mushroom residue + 1600 kg/hm² biochar (MDB1), 800 kg/hm² mushroom residue +3200 kg/hm² biochar (MDB2), and no mushroom residue or biochar added (CK). A field plot experiment was conducted to study the effects of different treatments on plant growth and yield, soil physicochemical properties and microbial community structure of Pandanus. Compared with CK, the yield increase effect of mushroom residue alone (MD) did not reach a significant level, while the biochar combined application treatments (MDB1, MDB2) significantly increased the fresh leaf yield of P. amaryllifolius. This was related to the significant increase in leaf SPAD value and net photosynthetic rate of MDB1 and MDB2, which significantly increased the average fresh leaf weight, but there was no significant difference in yield between MDB1 and MDB2 treatments. Soil nutrient results showed that compared with CK, MD, MDB1 and MDB2 all significantly increased soil pH and available potassium content, but MD treatment significantly reduced soil organic matter, alkali-hydrolyzable nitrogen and available phosphorus content, which may be related to the utilization of soil organic matter by a large number of organic matter-decomposing microorganisms in the mushroom residue leading to nutrient consumption, while the combined application of high-dose biochar (MDB2) alleviated this effect, thus increasing organic matter content and alkali-hydrolyzable nitrogen content. PCA analysis based on soil bacterial OTUs found that there were significant differences in community structure among different treatments. Compared with CK, MD soil bacterial Shannon diversity index decreased and Simpson dominance index increased, while MDB1 and MDB2 significantly improved soil bacterial diversity. Compared with MD, they significantly increased beneficial plant growth-promoting bacteria such as Pedomicrobium, Streptomyces, MND1, Nocardioides and Pseudomonas. RDA analysis showed that soil alkali-hydrolyzable nitrogen was the main environmental factor affecting bacterial community structure. In summary, although mushroom residue application alone has yield-increasing potential, its effect on soil improvement is poor; combined application of biochar can significantly increase soil pH, available potassium content, improve soil microbial diversity, promote the enrichment of beneficial microorganisms, thereby increasing leaf chlorophyll content, enhancing leaf photosynthesis, and ultimately promoting a significant increase in P. amaryllifolius yield. Therefore, the synergistic application of biochar and mushroom residue is an efficient and safe soil improvement strategy, providing green and sustainable technical support for efficient cultivation of Pandanus.