The invention discloses a cyclic dewfall system of a low-temperature liquid cargo ship. The system comprises a ship drying and humidity-reduction device, wherein one air outlet of the ship drying and humidity-reduction device is connected to each cargo cabin and each dry insulation cabin by pipelines; emptying holes of the cargo cabins and the dry insulation cabins are connected to a cargo cabin air return header pipe and a dry insulation cabin air return header pipe by branch pipe systems; the cargo cabin air return header pipe and the dry insulation cabin air return header pipe are connected to an air return adjustment device; the air return adjustment device is connected to an inlet end of the ship drying and humidity-reduction device; a bypass conversion valve is disposed between the ship drying and humidity-reduction device and the air return adjustment device; and one inlet of the bypass conversion valve is connected to a bypass new air pipe. According to the invention, gas cabin charging pipes and discharge pipelines of the cargo cabins/dry insulation cabins of the low-temperature liquid cargo ship are in sealed connection with a compressed drying system; air is absorbed and supplemented by the air return adjustment device based on a venturi tube; and the gas dew point at the inlet of the ship drying and humidity-reduction device can be reduced continuously and cyclically.