The invention relates to an automatic analyzer for determining the content of ammonia nitrogen in water and sewage through a flow injection colorimetric process and an automatic analytic method thereof. The automatic analyzer for determining the content of ammonia nitrogen in water and sewage comprises an automatic injection device, a peristaltic pump, a detector, and an online processing assembly, wherein the online processing assembly comprises a sample injection valve, braided reactors, three-way valves, a four-way valve, online heating modules, an online distillation module, an online degasser, and capillary connecting tubes. According to the automatic analyzer of the invention, samples can be continuously and automatically tested, the test speed is fast, fifty samples can be tested each hour, the test sensitivity is high, and the test accuracy is good; and according to the method of the invention, the liner range is 0.02-4.0mg / L, the liner correlation coefficient r is equal to or more than 0.999, the detection limit is 2.49mug / L, the standard addition recovery rate of practical water samples is 90-110%, the precision is high, and the repeatability is good.