Heavy metal contamination poses severe threats to human health. This study investigated the effects of exogenous silicon (Si) on cadmium (Cd) stress responses and underlying mechanisms in spinach and oilseed rape using hydroponic experiments. Three treatments were applied: control (CK), Cd stress (2 μmol·L-¹ Cd), and Cd+Si (2 μmol·L-¹ Cd + 0.5 mmol·L-¹ Si), with six replicates. Growth parameters, quality indices, antioxidant enzyme activities, and Cd content were analyzed. Si application significantly reduced Cd accumulation compared to Cd-stressed plants. Specifically, Cd content in spinach decreased by 31.6% (shoots) and 12.9% (roots), while in oilseed rape it decreased by 3.3% (shoots) and 28.6% (roots). Alleviating Cd stress, Si increased root length, plant height, and biomass in both species. Concurrently, soluble protein, soluble sugar, and vitamin C contents increased by 55.8%, 8.7%, and 5.7% in spinach and 14.1%, 14.9%, and 7.2% in oilseed rape, respectively. Si application also reduced malondialdehyde (MDA) content, an oxidative stress marker, by 23.8% (stem/leaf) and 17.1% (roots) in spinach, and by 23.5% (stem/leaf) and 20.4% (roots) in oilseed rape. Furthermore, Si modulated antioxidant enzyme activities. In spinach, it decreased superoxide dismutase (SOD) and catalase (CAT) in leaves, and peroxidase (POD), ascorbate peroxidase (APX), and CAT in roots, while increasing POD and APX in leaves and SOD in roots. Oilseed rape exhibited similar Si-induced decreases in leaf/root SOD, POD, and root APX, but showed increased leaf APX and leaf/root CAT activities. In conclusion, Si enhances Cd tolerance in spinach and oilseed rape by differentially modulating antioxidant enzyme activities and reducing oxidative damage, a mechanism not thoroughly elucidated in prior studies for these species. This work provides theoretical insights into Si-mediated Cd detoxification and practical implications for using Si amendments in safe agricultural production on contaminated soils.