Abstract: Under the use of mould fluxes with different fluorine contents, the change of fluorine ion (F^-) concentration in the continuous casting secondary cooling water was investigated in two plants. An experiment was also conducted to understand how the components and the soaking time of fluxes influence the F^-concentration of leaching water. The results showed that, when a flux with low fluorine content was used in Plant A, the increase of F^- concentration in the secondary cooling water was 1.2-24.3 mg·L^-1, and the water behaved in neutrality. While in Plant B, a flux containing high fluorine was used, the increase of F^- concentration in the secondary cooling water reached at 57.9 mg·L^-1, and the water behaved in acidity. The higher the fluorine content in the fluxes was and the longer the soak time of the fluxes in the water was, the more the fluorine would transfer into the water. Controlling the fluorine in the fluxes less than 2% in mass fraction was helpful to reduce the hazard effects of the mould fluxes on the secondary cooling water. In the researched composition ranges, the basicity (CaO/SiO₂) and the Li₂0 content had not an evident influence on the leaching of fluorine. Increasing the content of B₂O₃ could reduce the leaching of fluorine from the mould fluxes into the secondary cooling water.