The invention relates to a method for producing acid by fermenting excess sludge and synthesizing microbial oil, belonging to the field of resourceful treatment of sludge. The method comprises the following steps: (1) taking the excess sludge as a raw material, adding tea saponin into the excess sludge, and carrying out anaerobic fermentation to obtain a supernate; (2) removing ammonia nitrogen in the supernate obtained in the step (1) by adopting a struvite precipitation method, thus obtaining a solution containing volatile fatty acids; (3) by taking the solution obtained in the step (2) as a culture medium, culturing an oil-producing yeast by adopting a sequencing batch culturing method; and (4) by taking the yeast cells obtained after the culturing in the step (3) as a raw material, and extracting to obtain the microbial oil. Compared with the prior art, the low-concentration volatile fatty acids generated by the excess sludge are adopted as the culture medium for synthesizing the microbial oil with high content, and therefore, the environmental benefit and economic benefit brought by application of the excess sludge for producing acid are improved.