The five-leaf stage rapeseed Huayouza 62 （salt-tolerant variety） and Xiangyou 15 （salt-sensitive variety） were used to study the response of ion balance and photosynthetic characteristics in leaves of rapeseed seedlings to exogenous Ca²⁺ under alkali stress and to deeply understand the mechanism of adding exogenous Ca²⁺ to alleviate alkaline stress in rapeseed seedlings. Under NaHCO₃ （100 mmol/L） stress， six CaCl₂ levels including CK， 0%， 0.25%， 0.5%， 1%， and 2% （denoted as CK， T1， T2， T3， T4， and T5） were set to obtain appropriate concentrations of spraying CaCl₂ and to study their effects on ion balance and photosynthetic characteristics in leaves of rapeseed seedling. The results showed that rapeseed seedlings stored more Na⁺ in their leaves and accumulated more Ca²⁺ in their roots under NaHCO₃ stress. The imbalance of ions in Xiangyou 15 was more significant than that in Huayouza 62. When the concentration of CaCl₂ was 0.5%， the repair of plasma membrane damage under NaHCO₃ stress was most significant， which reduced the content of Na⁺ and increased the content of K⁺ and Ca²⁺ in the leaves of rapeseed seedlings. Except for the potential photochemical efficiency （F_v/F_o） and the maximum photochemical efficiency （F_v/F_m）， the inhibition of NaHCO₃ stress on the photosynthetic characteristics of Xiangyou 15 was greater than that of Huayuza 62. Spraying 0.5% exogenous CaCl₂ promoted the expression of chlorophyll a， chlorophyll b， total chlorophyll， F_v/F_o， F_v/F_m， net photosynthetic rate， stomatal conductance， and transpiration rate in seedlings. It is indicated that foliar spraying of CaCl₂ can regulate ion balance and photosynthetic characteristics of rapeseed seedlings under alkaline stress， and its improvement is more significant for the salt-tolerant variety.