Cadmium (Cd) pollution is one of the most extensive and harmful environmental problems worldwide. Recently, coastal water pollution by Cd and other heavy metals have been becoming more serious because of continuous economic development, including mining, urban traffic, burning of fossil fuels, and production of phosphate fertilizers. Calcium (Ca) is an essential macroelement that plays a crucial role in regulating plant growth and development. Moreover, Ca has been shown to alleviate heavy metal toxicity in many plants. Allium cepa is well known and commonly used in many laboratories primarily due to its many advantages, such as low cost, short test time, ease of storage and handling, large number of roots, and large cells with excellent chromosome conditions. In the present study, this plant was chosen as the experimental material to investigate whether the application of exogenous Ca can enhance the tolerance of A. cepa var. agrogarum L. to Cd stress (10,100 μmol/L and 300 μmol/L). In addition, the effects of various concentrations of exogenous Ca application (0, 0.1, 1, and 10 mmol/L) on the vegetative growth, photosynthesis characteristics, Cd accumulation, and nutrient minerals of A. cepa var. agrogarum L. under various concentrations of Cd stress (10 μmol/L to 300 μmol/L) were analyzed. This study showed the following results: (1) Excess Cd significantly inhibited the growth of A. cepa var. agrogarum L., resulting in changes in morphological characteristics, such as stunted and bent root tips and darkened green leaves, whereas exogenous Ca significantly decreased the negative effects of Cd and alleviated growth inhibition in A. cepa var. agrogarum L. (2) Cd treatment greatly increased the chlorophyll contents and significantly decreased net photosynthetic rate, stomatal conductance, and transpiration rate, whereas exogenous application of Ca markedly alleviated the increase in photosynthetic pigments and significantly increased the net photosynthetic rate, stomatal conductance, and transpiration rate. (3) Cd stress induced substantial Cd accumulation, resulting in serious nutrient imbalance, such as the significant inhibition of absorption of Mg (Magnesium ) and Mn (Manganese ) in roots and Ca, Mg, Mn, and Zn (Zinc ) in leaves and a massive accumulation of Ca, Fe (Iron) and Zn in roots and Fe in leaves, whereas the exogenous addition of Ca significantly decreased Cd accumulation and effectively reversed the Cd-induced imbalance of nutrients in A. cepa var. agrogarum L. Therefore, supplementing exogenous Ca with an optimal concentration can effectively relieve the phytotoxicity of Cd stress to A. cepa var. agrogarum L. by decreasing excessive Cd accumulation, maintaining high levels of photosynthesis characteristics, and maintaining the balance of water content, photosynthetic pigments, and nutrient elements, among others.