The utility model belongs to the field of seawater desalination, and relates to an 'outside-cylinder series-connection type' non-condensable gas removing device in a low-temperature multi-effect distillation seawater desalination system. The low-temperature multi-effect distillation seawater desalination system comprises evaporators, a steam injector, a steam condenser, the 'outside-cylinder series-connection type' non-condensable gas removing device, a concentrated saltwater heat exchanger, a fresh water heat exchanger, a seawater material feeding pump, a concentrated saltwater pump and a fresh water pump, wherein, the 'outside-cylinder series-connection type' non-condensable gas removing device comprises non-condensable gas communicating pipes, valves arranged on the communicating pipes, a vacuum pumping device, a connecting pipeline for connecting a non-condensable gas extracting opening of the steam condenser and the vacuum pumping device, and a valve arranged on the connecting pipeline; and each non-condensable gas communicating pipe is arranged outside a cylinder body of the corresponding evaporator and respectively connected with a heating steam side and a seawater evaporating side of the corresponding effect evaporator. The low-temperature multi-effect distillation seawater desalination system disclosed by the invention can adjust the non-condensable gas communicating pipes in an on-line manner and control the pressure difference among the effect evaporators, thereby adapting to working condition changes and guaranteeing the matching of the pressure of the effect evaporators.