Abstract: In this study, five cultivars of soybean, which were transgenic soybean by betaine aldehyde dehydrogenase (BADH) gene, non-transgenic parent 'Heinong35', wild soybean cultivar, local cultivar of 'Kangxianwang' and salt sensitive cultivar 'Hefeng 50', were planted in saline-alkali soil. The rhizospheric soil was sampled respectively at seeding stage, flowering and podding stage, pod-filling stage and maturity stage to determine (using classical methods) dynamic changes in bacteria population related to nitrogen transformation, biochemical function and available nitrogen contents. The results laid basic theory on the mechanisms of the effects of transgenic BADH soybean on nitrogen transformation. Compared with non-transgenic soybean, transgenic BADH soybean had positive effects on azotobacteria population and negative effects on ammonifying bacteria population at seedling, flowering and podding stages and with no significant effects on nitrifying bacteria population. Transgenic BADH soybean significantly increased nitrogen fixation intensity at maturity stage, and nitrification intensity at every growth stage of soybean. It also inhibited ammoniation intensity of soybean at seedling, flowering and podding, and pod-filling stages. The content of ammonium nitrogen in rhizospheric soils of transgenic BADH soybean was decreased compared with that of non-transgenic parent at seedling, flowering and podding stages. However, the content of ammonium nitrogen in rhizospheric soils with transgenic BADH soybean increased at maturity stage with no change at pod-filling stage. The content of nitrate nitrogen in rhizospheric soils of transgenic BADH soybean was higher than that of non-transgenic parent at seedling, pod-filling and maturity stages, and lower at flowering and podding stage. Transgenic BADH soybean influenced nitrogen transformation by changing the population of functional bacteria and the processes of biochemical intensity at seedling, flowering and podding stages.