CO cryogenic separation system and starting method thereof

The invention provides a CO cryogenic separation system and a starting method thereof. The cryogenic separation system comprises a process gas condensation unit, a hydrogen separation unit, a CO/CH4 separation unit and a nitrogen compression refrigeration unit, wherein the process gas condensation unit comprises a heat exchange device, a feed gas cooler and a synthesis gas liquid separation tank II; the hydrogen separation unit comprises a flash tower and a synthesis gas liquid separation tank I; the CO/CH4 separation unit comprises a CO/CH4 tower and a condenser; the nitrogen compression refrigeration unit comprises a nitrogen compressor; the CO product gas pipeline is communicated with the flash steam pipeline through a pipeline I, the CH4 product line is communicated with the flash steam pipeline through a pipeline II, and an outlet of the flash steam compressor is communicated with an inlet of the process gas condensing unit through a pipeline III. The method provided by the invention is short in start-up