NOVELTY - Starting a single-stage autotrophic denitrification reactor comprises inoculating sludge into a single-stage autotrophic denitrification reactor; constructing a nitrosobacteria and nitrobacterium microbiosystem under the conditions of limited oxygen supply and a small quantity of organic carbon source; controlling ammonium oxidation to nitrous acid stage; and optimizing a microenvironment with coexisting nitrosobacteria and anaerobic ammonium oxidation (anammox) bacteria through adjusting aeration mode and controlling dissolved oxygen level.    USE - Method for starting a single-stage autotrophic denitrification reactor, which is sequencing batch biofilm reactor.    ADVANTAGE - The method solves the difficulty of inoculum source of single-stage autotrophic denitrification reactor; and makes the starting mode simple and easy, and reduces the starting difficulty and the starting time of the single-stage autotrophic denitrification system.    DETAILED DESCRIPTION - Starting a single-stage autotrophic denitrification reactor comprises:    (A) inoculating sludge into a single-stage autotrophic denitrification reactor, where the inoculated sludge is mixed sludge of aerobic sludge and anaerobic sludge, the mixing volume ratio of aerobic sludge and the anaerobic sludge is 1-2:1, the total inoculating quantity is 60-70% of the effective volume of reactor, and the sludge is kept in a suspended state by stirring;    (B) constructing a nitrosobacteria and nitrobacterium microbiosystem under the conditions of limited oxygen supply and a small quantity of organic carbon source;    (C) controlling ammonium oxidation to nitrous acid stage through limited oxygen supply to enrich nitrosobacteria and inhibit the growth of nitrobacterium so as to establish a nitrosobacteria microbiosystem; and    (D) optimizing a microenvironment with coexisting nitrosobacteria and anaerobic ammonium oxidation (anammox) bacteria through adjusting aeration mode and controlling dissolved oxygen level on the basis of stable nitrous acid system to promote the growth and enrichment of the anammox bacteria to successfully start the single-stage autotrophic denitrification system.