The changes in the growth and physiological characteristics of soybean seedlings after the application of fomesafen were studied to provide a scientific basis for the rational application of herbicides in agricultural production. Accordingly, we carried out pot experiments to study the impacts of different dosages of fomesafen on soybean seedling growth and chlorophyll fluorescence parameters of soybean（SU18）. The results showed that when the dosage of fomesafen reached or exceeded 225 g a.i./ha, it significantly inhibited the growth of soybean root system, leaf peroxidase activity and the increase of malondialdehyde content. When the application dose of fomesafen exceeds 450 g a.i./ha, the photosynthesis of the leaves of soybean seedlings were destroyed, resulting in the decrease of soybean biomass. From the chlorophyll fluorescence imaging, the phytotoxicity of fomesafen on soybean photosynthesis gradually weakened with time. Excessive application of fomesafen can delay the overall growth and development of soybean seedlings and destroy photosynthesis in soybean seedlings.