Abstract: A novel composite passivator HAP−C was prepared by mixing hydroxyapatite (HAP) and hydrothermal carbon (HTC). The morphological characteristics, crystal structure, and functional groups of the prepared composite passivator were characterized using scanning electron microscopy and energy spectrum analysis, X ray diffraction, and Fourier transform infrared spectroscopy. Taking As contaminated soil as the research object, the effects of adding 1%, 3%, and 5% (mass fraction) of the composite passivator HAP−C on soil physicochemical properties, arsenic speciation, and enzyme activities were investigated. High-throughput sequencing technology was used to analyze the changes in soil microbial community composition. The results show that the prepared HAP−C contains the main characteristic elements of HAP and HTC (phosphate carboxylate and calcium) crystal structure and main functional groups (P—O,—OH,C=O,—CH2—). The addition of HAP−C can reduce the bioavailable adsorbed arsenic content in soil (22.20%-40.26%, mass fraction, the same below), and the passivation effect of 3% HAP−C is the most significant. The passivation mechanism of HAP−C to soil arsenic mainly includes surface complexation chemical precipitation and ion exchange. The addition of HAP−C not only increases soil pH (19.56%-21.51%), electrical conductivity (205.99%-290.91%), soil organic matter (23.12%-32.16%), available phosphorus (104.29%-176.54%) and nitrate nitrogen content (203.33%-250.19%), but also increases soil enzyme activity and microbial species richness and diversity. The relative abundance of Rhodanobacter and Sphingomonas in arsenic contaminated soil is relatively high. The relative abundance of Rhodanobacter and Gemmatimonas in 3% HAP−C treatment group is 10.30%-29.06% and 34.94%-48.12%, respectively, compared with the control group. The addition of HAP−C can not only enhance the passivation effect of As but also improve the health status of soil. HAP−C has a certain potential application in As contaminated soil remediation projects.