The invention discloses a continuous casting system diffusion chimney steam eliminating device, which comprises a dehydrater, an expansion section, a water cooling heat exchanger, an air cooling heatexchanger and a roof air cooling flow guide device, wherein the dehydrater is connected with a continuous casting secondary cooling indoor fan, so that high-temperature saturated steam enters the dehydrater; after the high-pressure spraying condensation, the temperature and the water content of the high-temperature saturated steam are reduced; the high-temperature saturated steam becomes unsaturated steam; the unsaturated steam enters the water cooling heat exchanger for heat exchange and then enters the air cooling heat exchanger for heat exchange defogging; the steam passes through a discharging flue to reach the roof air cooling flow guide device to be discharged to the periphery; dehydrated water flows back to the dehydrater. The invention provides a set of condensation, heat exchangeand defogging device; the saturated steam in the chimney is subjected to temperature reduction and condensation; oversaturated steam moisture in smoke gas is collected, so that the oversaturated steambecomes the unsaturated steam; the effect of eliminating the water content inside the steam is achieved; meanwhile, water dehydrated through condensation can be recovered and utilized; the effect ofreducing the water resource consumption is achieved.