Abstract: In order to reduce the odor pollution of septic tanks, reduce the number of septic tank cleanings, and reduce the operating costs of septic tanks, a composite microbial agent was prepared and its degradation application effect was explored. The composite microbial agent (solid preparation and liquid preparation) prepared by the research group, which was made of Bacillus tequilensis YZS-C01, Bacillus safensis YZS-C03, Alcaligenes faecalis QQB-A05 and Pseudomonas putida ZTB-A29, was used as the research object. The application effect was evaluated by measuring the in-situ reduction of fecal residue by its solid preparation and the removal rate of odor by its liquid preparation. The results showed that the solid preparation of the composite microbial agent could make all the scum in the septic tank within 10 days, and the degradation rate of fecal scum was above 65%, which was more than 60% higher than the settlement rate and degradation rate of the control group. When the liquid preparation of composite microbial agent was sprayed to treat the odor in septic tank, the instantaneous removal rate of ammonia and hydrogen sulfide was above 85%, and the removal rate of NH₃ and H₂S could till be maintained above 65% after 4 hours. It could also promote the degradation of scum in septic tank, with a degradation rate of 60%. The composite microbial agent prepared in the study could be used for the in-situ reduction of fecal scum and the removal of odor in septic tank at the same time, providing technical support for the in-situ reduction of fecal scum in septic tank and the bioaugmentation technology for odor treatment.