The invention relates to a low-purity oxygen air separating device and method. The device comprises an air compression system, a front precooling purification system and a cold box. The air compression system comprises a low-pressure air compressor and a high-pressure air compressor. The cold box comprises a main heat exchanger, a liquid oxygen evaporator, an expansion end of an expansion machine and a rectifying tower. The rectifying tower comprises a lower-pressure lower tower, a high-pressure lower tower, an upper tower, a low-pressure lower tower condensation evaporator and a condensation evaporator located between the upper tower and the high-pressure lower tower. Through the low-purity oxygen air separating device and method, the rectifying potential of the rectifying tower is sufficiently excavated, and compared with a conventional double-tower process, energy consumption is reduced by more than 10%; operation is easier than mixing of pure oxygen and air, and the product extraction rate reaches up to 99% or more.