Cadmium contamination of soil has been a world-wide concern in decades. Cadmium activity may vary sharply in submerged soils but the mechanisms so far found underlying the variation have remained conflicting and ambiguous because of the huge variety of the submerged soils, the complexity of their properties and the diversity of the external factors they are exposed. Various factors, such as Fe and Mn oxides, carbonates, particle organic matters, sulfide, DOM, anions (mainly Cl^- and SO₄^2-), cations (such as Al³⁺, Ca²⁺, Mg²⁺, Na⁺, etc.) and so on, and their controlling mechanisms not only perplex the soil chemistry of Cd, but also hamper the control and remediation of Cd polluted soils. At the oxic-anoxic interface of flooded soils, the variation of Cd activity is largely controlled by some chemical processes, such as adsorption-desorption, precipitation-dissolution, complexation and chelation. The paper reviews in detail these processes and mechanisms controlling the variation of Cd activity in flooded soils. At the same time, some important issues and directions are set forth for further research.