In order to optimize the start-up mode of anammox process, calcium ion was added to a sequencing batch reactor (SBR). The removal of nitrogen, the change of sludge activity and microbial colony during the start-up process were analyzed. The results showed that the addition of calcium ion reduced the start-up time of the anammox reactor by 8 days, increased the start-up efficiency by 25.8%, and enhanced the denitrification stability of the reactor under high nitrogen load. Calcium ion could inhibit the growth of acid-producing bacteria and filamentous bacteria, but promote the enrichment of anammox bacteria. The abundance of Candida Brocadia reached 14.37% in the stable period, and this kind of bacteria could promote the coexistence of nitrifying bacteria, denitrifying bacteria and anammox bacteria.