Using molecular sieve as adsorbent, the plant separates oxygen and nitrogen from the air basing on the difference of adsorption quantity of oxygen and nitrogen at the surface of molecular sieve via pressure change. For basing on natural air separation, it features low dissipation of energy, fast startup, (meeting requirements within 30 minutes), high automation, compact conformation, small footprint, and one-time investment in small amounts. The purity of oxygen using pressure swing adsorption method hits 93% and the purity of nitrogen reaches 99. 5% - 99. 99%, suitable for industries that do not require very high purity, such as metallurgy and chemical industry.