The Ferrous iron can be released due to the acid solubility of FeS which is ubiquitous in nature. Therefore, we studied the effects of FeS on nitrogen removal performance and microbial community of anammox process. The start-up time of the anammox reactor was shortened and the total nitrogen removal rate was significantly enhanced with the addition of FeS. FeS promoted the formation of anammox granular sludge and stimulated the synthesis of the heme c. The addition of FeS increased the abundance of anammox bacteria, but did not change the dominance of Ca. Kuenenia genus. In addition, the relative abundance of denitrifying bacteria in the community was reduced obviously with the addition of FeS, possibly due to the inhibitory effect of S2- on nitrous oxide reductase and nitric oxide reductase. This study demonstrated the feasibility and advantages of the FeS addition in the anammox process and provided an alternative option for the mainstream application of anammox.