A method for preparing high purity ammonia is provided, which comprises the following three steps of: (1) obtaining the required feed gases (i.e., hydrogen-nitrogen gas mixture) by catalytic cracking ammonia; (2) purifying the hydrogen-nitrogen gas mixture; and (3) synthesizing high purity ammonia by using the hydrogen- nitrogen gas mixture with high purity. In the provided method, the obtained ammonia with undesired purity is fed back to an ammonia catalytic cracking unit. The whole production system is a closed system without any discharging of ammonia and thus is environment friendly. Each step of the method can reduce cost.