Calcium (Ca²⁺) is an essential nutrient for plants and enhances their resistance to heavy metal stress. This study employed hydroponic experiments to investigate how varying Ca²⁺ levels (0, 0.5, 2, and 5 mM) regulate maize seedling growth, physiological-biochemical traits, and cellular antioxidant defense systems under cadmium (Cd) stress, thereby alleviating Cd toxicity. The results showed that Ca²⁺ supplementation mitigated the inhibitory effects of Cd stress on root length, dry weight, and photosynthetic pigment content. Additionally, Ca²⁺ application increased the activities of superoxide dismutase (SOD) and catalase (CAT), as well as the contents of glutathione (GSH) and ascorbic acid (ASA) in leaves, with maximum increases of 14.9%, 65.39%, 146%, and 135%, respectively, compared to the control (CK). Under Ca²⁺ treatment, peroxidase (POD) activity significantly decreased, with a maximum reduction of 34.38% compared to CK, while malondialdehyde (MDA) content markedly increased, reaching 19.84% higher than CK. Principal component analysis (PCA) revealed that the 5 mM Ca²⁺ treatment achieved the highest composite score, indicating optimal antioxidant capacity in maize seedlings and reduced membrane lipid peroxidation caused by excessive Cd accumulation. This study enhances the understanding of calcium’s role in plant stress responses and provides insights for safe maize production in Cd-contaminated environments.