The invention relates to an ammonia nitrogen removal agent and a method for treating sewage by utilizing the agent, relates to the field of sewage treatment, and provides the ammonia nitrogen removalagent capable of improving a denitrifier propagation ability and with high shock resistance, and a method for treating the sewage by utilizing the agent. The ammonia nitrogen removal agent is preparedfrom the following components: 8 to 10 parts of arthrobacter globiformils, 15 to 20 parts of paracoccus denitrification, 10 to 12 parts of bacillus subtilis, 8 to 10 parts of nocardia amarae, 15 to 20 parts of nitrobacter winogradsky, 20 to 25 parts of pseudomonadaceae K50, 20 to 25 parts of pseudomonadaceae TR2, and 2 to 5 parts of snail stoste. The appropriate cultures are matched reasonably, so that a nitration-denitrification effect achieves the best; the snail slime is added into the agent, so that the cultures are promoted to grow and propagate, an ammonia nitrogen treatment effect is improved, and the shock resistance is improved; coconut oil fatty acid diethanol amide converts ammonium at an ion state into free ammonia, so that a denitrification progress of whole denitrification culture is accelerated; in addition, only N2 is produced during a denitrification process, and the production of N2O products in a sewage treatment system is reduced, so that the environment is protected.