The invention provides a fractionation tower online salt leaching system and technology, and belongs to the technical field of crude oil refining. The online salt leaching system comprises a water washing water input pipeline (4), a water collecting device (3) arranged between two trays in a fractionation tower (1) and used for collecting salt-containing sewage, a first salt-containing sewage output pipeline (8) and an oil-water separation tank (2), wherein the water collecting device (3) comprises a water collecting tank (7), a downcomer (5) and gas rising holes (6), the collecting tank (7) comprises an concave part (701) and a horizontal part (702), the concave part is connected with the first salt-containing sewage output pipeline (8), the downcomer (5) is arranged at the concave part (701) of the collecting tank (7), and the gas rising holes (6) are arranged at the horizontal part (702) of the collecting tank (7). The online salt leaching system can be used to clean a large quantity of trays without generating polluted oil.