The invention relates to a method for shifting ammonia in a diphenyl urea reaction system by nitrogen. A nitrogen pipeline with the pressure resistance of 10-20kg/cm<2> is introduced into a reactor from the upper part of the reactor; a gas stripping outlet of the nitrogen pipeline is arranged between the lower part of a stirring paddle and the bottom of the reactor; the gas stripping outlet formshole shrinkage; the diameter of a hole ranges from 100 microns to 1mm; the nitrogen flow is 3 to 5 times of reactor size per hour; the reaction nitrogen charging time is 2-4 hours; under the same process condition, the consumption of nitrogen is reduced by 4-6L/kg DPU compared with the traditional method; and the diphenyl urea yield is improved to 96-99 percent by more than 6 percent. The invention has the advantages of high gas stripping efficiency and difficult blockage of the gas stripping outlet of the nitrogen pipeline.