The invention discloses a CO cryogenic separation system using double circulation. The CO cryogenic separation system comprises main heat exchangers, a low-temperature separator, a hydrogen stripping tower, a denitriding tower, a demethanizer, a CO compressor and a circulation nitrogen compressor. The invention further discloses a separation method of the CO cryogenic separation system using double circulation. The method includes the processing steps of heat transfer, dehydrogenation, denitrification, demethanation, CO compression cycle, nitrogen compression refrigeration cycle and the like. The CO cryogenic separation system using double circulation and the separation method thereof have the beneficial effects that the double-circulation process of CO compression and nitrogen compression is adopted, and the dual advantages that rectification energy consumption of CO compression cycle is low and compression energy consumption and investment of nitrogen cycle are low are given into play. A heat source of a rectifying tower evaporator and a cold source of a condenser are optimized, and rectification loads can be controlled reliably and stably. Starting and purifying time of the device can further be shortened, and the evacuation amount at the starting stage is decreased. Energy consumption is low, the starting speed is high, the investment can be reduced, and the quantity of emissions is small. The general trend of energy saving and cost reduction are met, and the economic benefits and environmental protection benefits are good.