Abstract: Annual vine Glycine soja Sieb. et Zucc. is considered the ancestor of cultivated soybean (G. max (L.)Merr.), but exhibits greater genetic diversity. At present, the G. soja growth environment is under high-salt stress, but its photosynthetic performance under such conditions remains unknown. In this study, we investigated the effects of salt stress on the photochemical activity of G. soja photosynthesis based on prompt chlorophyll fluorescence and modulated 820-nm reflection. Results showed that chlorophyll a content was significantly reduced, and the chlorophyll fluorescence induction transient (OJIP) curve was significantly changed in seedling leaves after salt stress treatment. The JIP-test parameters, including performance indices such as PI_ABS and PI_total and energy flux parameters such as RC/ABS, TRo/RC, ETo/RC, and REo/RC, were decreased, while DIo/RC was increased. Quantum yield and efficiency parameters, such as ψEo, φEo, δRo, and φRo, were decreased in seedling leaves exposed to salt stress. The shape of the MR/MR_O ratio curve changed after salt stress treatment. Furthermore, changes in the MR/MR_O ratio showed high correlation to the time intervals of chlorophyll fluorescence. Salt stress led to membrane lipid peroxidation in the seedling leaves, resulting in a significant increase in MDA content, while relative water content was significantly decreased. Thus, the seedling leaves adapted to salt stress by significantly increasing osmotic regulators and antioxidant enzyme activities.