Abstract: Using steel slag as a conditioner and sludge for mixed composting, the effects of steel slag on composting temperature, moisture content, contents of organic matter and ammonium nitrogen were studied. The variation characteristics of bacterial community during composting were analyzed by microbial high-throughput sequencing technology, and the correlation between environmental factors and bacterial community was evaluated by redundancy analysis. The results show that the addition of steel slag can increase the compost temperature, extend the high temperature maintenance time and reduce the moisture content, promote the conversion of ammonium nitrogen in the sludge, which is beneficial to reduce nitrogen loss and increase the degradation rate of organic matter; steel slag affects the bacterial community structure and diversity in the compost, it can increase the relative abundance of Firmicutes and Actinomycotas in the later stage of composting, and promote the decomposition of cellulose in the pile, the diversity and richness of bacteria in compost containing 15% steel slag are the highest. Temperature, moisture content, organic matter, ammonium nitrogen and other factors were positively correlated with the bacterial community structure, and adjusting the above factors can effectively improve the composting quality.