Abstract: To clarify the effects of parent material and rice cropping season on the distribution of iron forms in paddy soils and their coupling relationships with cadmium (Cd) activity, three typical paddy soils in Hunan Province were selected: red clay paddy soil derived from Quaternary red clay, sandy clay paddy soil derived from granite weathering products, and eel mud paddy soil derived from slate weathering products. X-ray diffraction (XRD) characterization, sequential chemical extraction, and correlation analysis were used to investigate the effects of parent material and rice cropping season on iron form distribution and Cd activity. The results showed that the iron mineral composition of soils derived from different parent materials varied significantly, which had important effects on Cd activity. Among the three soils, the eel mud paddy field, containing both goethite and lepidocrocite, exhibited the strongest ability to inhibit the transformation of Cd into labile forms; the red clay paddy field, with a high content of free iron (Fe_d), showed intermediate inhibition; and the sandy clay paddy field, although rich in organic matter, displayed the weakest inhibition due to its low Fe_d content and sandy texture. Parent material significantly influenced the soil iron pool characteristics, with the red clay paddy field having the highest contents of Fe_d and Fe(II), reaching 8.94 g/kg and 2.76 g/kg, respectively. The rice cropping season drove iron form transformation by regulating soil water regime. The long-term flooding during the early rice season created a reducing environment, resulting in higher contents of Fe(II) and chelated iron (Fe_p); whereas field drying during the late rice season enhanced oxidizing conditions and promoted the formation of amorphous iron oxide (Fe_o). Correlation analysis showed that under acidic conditions, Fe_d exhibited a highly significant negative correlation with labile Cd, and both Fe_p and Fe_o were negatively correlated with labile Cd, indicating that different iron forms collectively participated in Cd fixation through processes such as adsorption, coprecipitation, and complexation. The study highlights that evaluation of iron oxide effects on Cd activity should consider not only content but also crystallinity, mineral composition, and soil physicochemical properties. Parent material and rice cropping season jointly regulate iron form transformation and thereby influence Cd activity distribution, improving understanding of Fe–Cd coupling in waterlogged paddy systems.