The invention provides an external cooling type low pure oxygen air separation system and method. The external cooling type low pure oxygen air separation system comprises a lower tower, a condensing evaporator, an oxygen inlet, an oxygen heat exchanger, a nitrogen heat exchanger, a throttle valve and a delivery pipeline; the condensing evaporator is arranged at the top of the lower tower; the throttle valve is arranged on an outer side of the lower tower; liquid oxygen is fed into the condensing evaporator through the oxygen inlet; raw material air is precooled by the heat exchanger to reach a saturation temperature before going into the bottom of the lower tower, and nitrogen in the raw material air rises to the top of the lower tower and goes into the condensing evaporator; one part of the nitrogen exchanges heat with the liquid oxygen in the condensing evaporator and thus is condensed into liquid nitrogen for flowing black, while the other part of the nitrogen is discharged out of the condensing evaporator and reheated by the nitrogen heat exchanger to reach a normal temperature before being recovered; the liquid oxygen is transformed into oxygen that is discharged out of the condensing evaporator and reheated by the oxygen heat exchanger to reach the normal temperature before being recovered; the liquid oxygen staying at the bottom of the lower tower is discharged out of the lower tower and delivered into the condensing evaporator at the top of the lower tower through the delivery pipeline with the throttle valve to serve as a cold source taking part in the heat exchange process.