NOVELTY - Immobilizing salt-tolerant cold-tolerant ammonia oxidizing bacteria using biochar ball as carrier comprises e.g. filtering out high efficiency ammonia oxidizing bacteria, then carrying out cultivation of ammonia oxidation bacteria to obtain salt-tolerant cold-tolerant ammonia oxidation bacteria, adding biochar ball in salt-tolerant cold-tolerant ammonia oxidation bacteria enriched liquid to obtain biochar ball biofilm, embedding biofilm biochar ball and salt-tolerant cold-tolerant ammonia oxidation bacteria in mixed solution, placing in saturated boric acid solution.    USE - The method is useful for immobilizing salt-tolerant cold-tolerant ammonia oxidizing bacteria using biochar ball as carrier. The bacteria is used in high salt condition ammonia nitrogen polluted water body purification, and low temperature and high salt condition ammonia nitrogen polluted water body purification (claimed).    ADVANTAGE - The method uses available raw material source, has simple preparation process, is easy to industrialize. The strain has high mechanical intensity, provides good treatment effect in water body purification, is economical, no need to recover, is environmentally friendly environment, and does not cause secondary pollution in water body purification.    DETAILED DESCRIPTION - Immobilizing salt-tolerant cold-tolerant ammonia oxidizing bacteria using biochar ball as carrier comprises filtering out high efficiency ammonia oxidizing bacteria, then carrying out cultivation of ammonia oxidation bacteria to obtain salt-tolerant cold-tolerant ammonia oxidation bacteria, adding biochar ball in salt-tolerant cold-tolerant ammonia oxidation bacteria enriched liquid to obtain biochar ball biofilm, embedding biofilm biochar ball and salt-tolerant cold-tolerant ammonia oxidation bacteria in mixed solution, taking out from grain by grain, placing in saturated boric acid solution, using the biologic char ball in mixture liquid, placing in grain by grain in saturated boric acid solution, placing in the refrigerator of 4 degrees C for 12 hours, crosslinking using sterile water to clean to obtain salt resistant and cold resistant ammonia oxidizing bacteria immobilized biochar ball.