<p>Free-living nitrogen-fixing bacteria (FNFB) can help plants absorb heavy metals in soils via their function of fixing nitrogen. Here, we investigated the difference of <i>nifH</i> gene diversity of culturable free-living nitrogen-fixing bacteria (CFNFB) from soils cultured with <i>Robinia pseudoacacia</i> L. (black locust) and <i>Medicago sativa</i> L. (alfalfa) under cadmium (Cd) exposure. The <i>nifH</i> gene was selected to explore the divergence of CFNFB community from soils cultured with the two plants under Cd exposure (2, 4, 7, 15, and 20&#xa0;mg Cd·kg^− 1 dry soil). The Shannon and Simpson indices increased significantly at 4&#xa0;mg Cd kg^− 1 dry soil compared to 15&#xa0;mg Cd kg^− 1 dry soil in soils cultured with black locust, however, there was no significant difference in alpha diversity of <i>nifH</i> gene in soils cultured with alfalfa between different Cd treatments. <i>Ideonella</i>, <i>Bacillus</i>, and <i>Bradyrhizobium</i> were common genera living in soils of two plants, which were the top three dominant genera presenting under all Cd levels. Furthermore, the relative abundance of <i>Ideonella</i> was the highest in soils cultured with the two plants. Cd had a significant effect on community structure of CFNFB from soils cultured with alfalfa according to NMDS and LefSe data. Plant species significantly affected CFNFB community in Cd-contaminated soils. As a culturable dominant genus with the highest abundance under Cd treatments irrespective of plant species, <i>Ideonella</i> could be expected to be a resource for constructing nitrogen-fixing microbial fertilizers for bioremediation of Cd-contaminated soils. Overall, our findings will provide some strategies for the selection of excellent strains that can promote phytoremediation of heavy metal-polluted soils.</p>