The objective of this study is to investigate the effects of salt stress on seed yield, quality, and physiological processes in rapeseed. During rapeseed growing season from 2020 to 2022, two different soil salinity levels of low soil salinity (LS) and high soil salinity (HS) were conducted for rapeseed planting in Dafeng city, Jiangsu province, China (33°24′N, 120°35′E). The results indicated that, compared with LS treatment, the biomass accumulation under HS treatment was decreased by 18.46%-35.67% at the early flowering stage, and 20.92%-46.03% at maturity stage, respectively. HS treatment increased the proportion of dry biomass distribution in roots and leaves and decreased the proportion of stem and branch distribution at the early flowering stage, and increased the proportion of root, stem, and branch distribution and decreased the proportion of pod and seed distribution at maturity stage. Moreover, compared with LS treatment, HS treatment decreased the accumulations of carbon (C) and nitrogen (N) in various organs at both early flowering and maturity stages, and decreased the efficiency of C and N translocation in stems, branches, and leaves at reproductive stage, indicating that salt stress inhibited C and N assimilation and translocation, and ultimately led to a decrease in seed yield. Besides, C/N in all organs under HS treatment was lower than that under LS treatment, indicating that the adverse effects of salt stress on C assimilation were more intensive than those on N assimilation, which resulted in the increased seed protein content by 8.23% and decreased oil content by 4.42%. In addition, compared with LS treatment, HS treatment decreased net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and instantaneous carboxylation efficiency (CE); however, it increased intercellular CO2 concentration (Ci) and water use efficiency (WUE), which indicating that the effects of salt stress on rapeseed photosynthesis were mainly attributed to the non-stomatal factors. HS treatment increased peroxide (H2O2) and malondialdehyde (MDA) content by 27.41% and 42.33% compared with LS treatment. The superoxide (SOD) activity, catalase (CAT) activity, ascorbic acid (AsA) content, soluble protein content, and soluble sugar content under HS treatment were increased by 65.54%, 22.85%, 29.68%, 9.75%, and 16.84%, compared with LS treatment, respectively. In conclusion, salt stress decreased the yield and changed quality by inhibiting C and N assimilation and photosynthesis in rapeseed, which could improve the antioxidants and osmotic regulation ability to adapt to salt stress environment.