The GlnK protein is a PⅡ family protein existing widely in bacteria,archaea and plants and involved in regulating nitrogen metabolism. The insertion mutant of glnK homologue gene SfHH103_03182 in Sinorhizobium fredii HH103 was constructed. The results of PCR and sequence analyses confirmed that the mutant was correctly constructed. The results of pot plant experiments showed that the nodule number of the plant inoculated with the mutant,the fresh weight of plants and the fresh weight of root nodules increased compared with inoculated with wildtype strain HH103. There was no significant difference in root nodule nitrogenase activity. The results of symbiotic phenotype tested on host plant soybean at different concentrations of nitrogen levels showed that there was no significant changes in the number of root nodules in the mutant inoculated plants.