The energy consumption of an ammonia process is huge, and it occupies a large proportion in the industrial energy consumption. The air separation unit is an important energy-consuming unit in the process of ammonia synthesis. For the ammonia process with pure oxygen gasification, the purity of oxygen products in the air separation unit is generally 99.6%. If the oxygen purity from the air separation is reduced properly, its energy consumption will decrease effectively, and the energy consumption of the whole ammonia process may further decrease, as the nitrogen element is needed for ammonia synthesis. However, with the increase of the flow rate, the operating cost of the subsequent unit would increase, so there is an optimal purity that could minimize the energy consumption of the whole process. In this paper, the process simulation software, ASPEN PLUS, is used to model and analyse the process of ammonia synthesis to determine the process energy consumption with different inlet oxygen purity. With the reduction of oxygen purity, the oxygen compressor power consumption increases, nitrogen compressor power consumption decreases, and the refrigeration energy consumption of the purification unit increases. The relations between the three kinds of energy consumption and oxygen purity are linear. Combined with the relation between oxygen purity and the energy consumption of air separation unit, the optimal oxygen purity that makes the process energy consumption minimal is 92%. Compared with the current process used 99.6% purity oxygen, the energy saving is about 13.3% as much as air separation energy consumption.