Contamination of cadmium (Cd) in paddy soil is a serious environmental problem threatening food safety in some parts of southern China, where rice grain Cd concentration often exceeds the Chinese limit (0.2 mg kg-1). Many field studies have shown a poor relationship between rice grain Cd concentration and soil total Cd concentration and other soil properties. One of the reasons is that the bioavailability of Cd in paddy soil is strongly influenced by the soil redox potential, especially during the grain filling stage. To investigate this effect, we conducted field experiments in a paddy field contaminated with a moderate level of Cd (total soil Cd 2.6 mg kg-1, pH 5.8) in Xiangtan, Hunan province in 2017 and 2018. The paddy soil surface was prepared in a slight slope (5 cm) along the 20 m length from the irrigation inlet to the outlet. This slope created a gradient of soil water content and hence, the redox potential, when paddy water was drained during the grain filling period. Soil Eh, pH, and dissolved Cd concentration were monitored during the rice growing season. During the grain filling stage when paddy water was drained from the outlet, soil Eh ranged from -200 to -100 mV in the outlet area to 100 – 200 in the inlet area, whilst soil pH varied from 6.2 to 6.7. At maturity, grain Cd concentration showed a distinct spatial pattern, increasing from low levels in the outlet area to high levels in the inlet area, with a difference of 5 fold. To test if the gradient in soil Cd bioavailability generated from differential redox potential along the slope was caused by the drainage-induced variation in soil pH, we applied lime (4.5 t ha-1 CaCO3) to one of the two adjacent field slopes. Liming increased soil pH the initial 5.8 to 6.5. In the unlimed control, rice grain Cd concentration showed the same gradient of 6-fold variation. In contrast, grain Cd concentrations in the limed slope were generally very low and showed no clear spatial pattern. Our results show that variation in the paddy water status during the grain filling can cause a large variation in grain Cd concentration, and this effect is attributed mainly to soil pH changes associated with the variation in the redox potential. To decrease grain Cd concentration, paddy field should be maintained flooded as late as possible during grain filling or that paddy soil should be limed to pH above 6.5.