The invention discloses a stay cable dehumidification system and a stay cable dehumidification method. The stay cable dehumidification system comprises a dehumidification module as well as a stay cable ventilation module which is used to convey new air processed by the dehumidification module to a stay cable. The dehumidification module comprises a filter device, a dehumidification device, a dry air balance device and a gas conveying device, wherein the filter device is used to filter the input new air; the dehumidification device is used to dehumidify the new air filtered by the filter device; the dry wind balance device is used for gas pressure balance of the new air dehumidified by the dehumidification device; and the gas conveying device is used to make the new air be conveyed to the filter device, the dehumidification device and the dry wind balance device in succession. According to the stay cable dehumidification system and the stay cable dehumidification method provided by the invention, dry wind is conveyed to the stay cable by the dehumidification system, so that dehumidification and corrosion prevention can be achieved; the system can be detected in time; corrosion problems of upper and lower anchor heads and the stay cable are solved; replacement frequency of the stay cable is greatly reduced; and bridge safety is enhanced effectively.