Abstract: In order to optimize the start-up mode of anammox process,calcium ion was added to a se-quencing batch reactor ( SBR) . The removal of nitrogen,the change of sludge activity and microbialcolony during the start-up process were analyzed. The results showed that the addition of calcium ionreduced the start -up time of the anammox reactor by 8 days,increased the start -up efficiency by25.8%,and enhanced the denitrification stability of the reactor under high nitrogen load. Calcium ioncould inhibit the growth of acid - producing bacteria and filamentous bacteria,but promote theenrichment of anammox bacteria. The abundance of Candida Brocadia reached 14.37% in the stableperiod,and this kind of bacteria could promote the coexistence of nitrifying bacteria,denitrifyingbacteria and anammox bacteria. Close-