The microstructure, tensile properties, and impact toughness of niobium-stabilized austenitic stainless steel with Si content of 0.076wt%–3.80wt% were characterized by OM, SEM, and TEM. The results show that variations in Si content exert minimal influence on grain size and primary NbC, and the formation of δ-ferrite is promoted as the Si content reaches 3.80wt%. The planar slip of dislocation is the dominant deformation mechanism of the three steels, and deformation-induced martensitic transformation is induced by the high local plastic strain in the slip band. The increase in Si content promotes the planar slip of dislocation, and the local plastic strain in the slip band is decreased, resulting in the suppression of the deformation-induced martensitic transformation. As a consequence, the transformation from massive martensite to fine lath martensite is promoted with the increase in Si content. The enhancement effect of Si addition on the tensile strength is due to the secondary strain hardening, which is caused by dislocation plane slip and deformation induced martensitic transformation. However, the increased probability of secondary crack formation at the NbC/austenite interface leads to the decrease in elongation. The increase in Si content reduces impact toughness. The decrease in slip band spacing can increase the nucleation sites of void, resulting in a decrease in crack initiation energy. At the same time, the increase in the number of martensite/austenite interfaces leads to a decrease in crack propagation energy.