The invention relates to the field of environment protection biology and particularly relates to a multistage-amplified full-automatic on-line cultivation enlarging device of nitrobacteria and a cultivation enlarging method thereof. A cultivation enlarging tank is respectively connected withconnected to a primary seed tank and a bacteria solution temporary storage tank; a nitrogen source tank is respectively connected withconnected to thea primary seed tank and a cultivation enlarging tank through a nitrogen source pipeline; a carbon source tank is respectively connected withconnected to the primary seed tank and the cultivation enlarging tank through a carbon source pipeline; an air compressor is respectively connected withconnected to the primary seed tank and the cultivation enlarging tank through an air pipeline; stirring devices are respectively mounted on the primary seed tank and the cultivation enlarging tank; jackets are respectively arranged on the tank bodies of the primaryseed tank, the cultivation enlarging tank and the bacteria solution temporary storage tank; the jackets of the primary seed tank and the cultivation enlarging tank are respectively connected withconnected to a constant-temperature water tank; the jacket of the bacteria solution temporary storage tank is connected withconnected to a water chilling unit; the temperature of each jacket is fed back toan automatic control system; the automatic control system is respectively connected withconnected to the stirring devices, the constant-temperature water tank and the water chilling unit. The devicedisclosed by the invention can be applied to industrial cultivation enlargement of nitrobacteria and solves the problem of ammonia nitrogen treatment of wastewater.