Taking cucumber cultivar ‘Xinchun 4’ as experimental material, we studied the effects of exogenous spraying 0.01 mg/L exogenous 2,4epibrassinolide (EBL) and 24 μmol/L brassinolide inhibitor (BZR) on the rapid chlorophyll fluorescence induction kinetics curve (OJIP) and related fluorescence parameters of cucumber seedlings under moderate salt stress (50 mmol/L NaCl), and discussed the photosynthetic physiological mechanism of EBL alleviating the damage of cucumber seedlings under moderate salt stress. The results showed that: (1) salt stress led to the decrease of net photosynthetic rate (P_n), stomatal conductance (G_s) and transpiration rate (T_r), the increase of intercellular CO₂ concentration (C_i), the decrease of initial fluorescence (F_o) and maximum fluorescence (F_m), and the increase of J point and I point in the OJIP curve, which reduced the photosynthetic performance of cucumber seedlings, and the damage to PSⅡ receptor side was greater than that on the donor side, showing the damage of PSⅡ reaction center. The transfer efficiency of photosynthetic electron from Q_A to Q_B decreased, and the electron transfer was blocked. (2) Under 50 mmol/L NaCl treatment, exogenous spraying of 0.01 mg/L EBL could significantly improve P_n, G_s, T_r, and photosynthetic performance (PI_ABS) of cucumber seedlings, reduce C_i, and significantly increase the absorption per unit area (ABS/CS_m), capture (TR_o/CS_m), light energy for electron transport (ET_o/CS_m), and the number of active reaction centers (RC/CS_m). (3) Compared with NaCl + EBL treatment, the photosynthetic performance of cucumber seedlings after NaCl + EBL + BZR treatment was further reduced, indicating that EBL could alleviate PSⅡ damage caused by salt stress in cucumber seedlings. It was found that exogenous spraying of appropriate amount of 2,4epibrassinolide could effectively alleviate the damage of (PSⅡ) receptor side of photosynthetic electron transport chain on cucumber seedling leaves, increase the efficiency of electron transfer from Q_A to Q_B, and thus significantly improve the photosynthetic performance of cucumber seedling leaves under salt stress.