In order to seek for the reasons why one-stage RO of seawater is corrosive to Q235 carbon steel, dynamic coupon tests were carried out to research on the corrosion properties of Q235 carbon steel in seawater and one-stage RO of seawater, and the surface appearance of plates and corrosion components were analyzed. Results indicate that, corrosion rate of Q235 carbon steel in seawater declines gradually with corrosion time going and approaches to steady; While in one-stage RO of seawater, corrosion rate increases and becomes steady gradually. The components of corrosion products in these two water bodies are same, but their relative content and change tendency are quit different. The production in RO water, which is ball-shaped, is less compact than the needle-shaped production engendered in sea-water. The weak acidity of RO water promotes the transform ofγ-FeOOH to conductive Fe3O4. The rust is disconnected and can’t impede the transition of oxygen. These are the reasons leading to high-speed corrosion of Q235.