In order to explore the effects of nicotinamide on salt and alkaline tolerance of soybean seedlings and its physiological mechanism,this paper used soybean Hefeng 50 as experimental material to analyze the growth indicators and physiological indicators of soybean seedlings under different concentrations of nicotinamide sprayed under salt and alkali stress, as well as the expression level of nicotinamide enzyme genes (NIC1) under different treatments. The results showed that a certain concentration of niacinamide treatment could increase the plant height, root length, and biomass of soybean seedlings under salt alkali stress, while also increasing the content of osmoregulatory substances, antioxidant enzyme activity, and photosynthetic pigment content in leaves. Under the experimental conditions,the best effect was achieved when the mass concentration of niacinamide solution was 50 mg/L. After nicotinamide treatment, the expression level of NIC1 gene in soybean leaves was upregulated, indicating that the upregulation of nicotinamide enzyme gene could improve the salt alkali tolerance of soybean. The research results provided a reference for the physiological mechanism of nicotinamide regulating salt and alkaline tolerance in soybean seedlings.