The invention discloses a method for rapidly removing low-concentration ammonia nitrogen from polluted river water. The method comprises the following steps: (1) conveying the polluted river water into a mixing tank; adding an oxidant into the mixing tank and continuously stirring and mixing for 60 to 80 seconds; (2) introducing river water to be treated into an ultrasonic strengthening reaction tank; controlling the frequency of ultrasonic waves to 5 to 20KHz, the sound intensity to 0.1 to 1.0W/cm<2> and the hydrolytic retention time to 30 to 60 seconds; (3) conveying output water from the ultrasonic strengthening reaction tank into a rotary reaction tank and continually reacting for 6 to 8 minutes; (4) conveying output water from the rotary reaction tank into an air floatation tank; removing pollutants in the water through an air floatation technology; finally, obtaining clean river water without the ammonia nitrogen. According to the method disclosed by the invention, a combining scheme of breakpoint chlorination oxidization treatment and ultrasonic treatment is adopted, so that the reaction efficiency is improved; the ammonia nitrogen is efficiently removed and the operation cost is saved.