Black locust (Robinia pseudoacacia L.) is a pioneer legume for afforestation in soft rock areas of the Loess Plateau, China. Our study aimed to assess the effects of Glomus mosseae and Rhizobium strain NWYC129 on the growth of black locust seedlings and the quality of weathered soft rock soils under pot conditions. Black locust seedlings could form strong symbiotic relationships with arbuscular mycorrhizal (AM) fungi and rhizobia. Dual inoculation with G. mosseae and Rhizobium NWYC129 had the best effects on shoot dry weight, root dry weight, shoot height, root length and stem diameters of black locust seedlings, compared with single and non-inoculation. Inoculation with G. mosseae and/or Rhizobium NWYC129 clearly enhanced the content of organic matter in weathered soft rock soils, and significantly improved the transport rate of soil phosphorus from the rhizosphere soils to plant tissue compared with non-inoculation. Treatments inoculated with Rhizobium had higher available nitrogen concentrations in weathered soft rock soils than treatments non-inoculated with Rhizobium. Dual inoculation showed the greatest activities of urease, alkaline phosphatase, invertase and catalase among the five treatments. The content of Bradford-reactive soil proteins (BRSP) was increased by inoculation with G. mosseae, and the highest contents of easily extractable Bradford-reactive soil proteins (EE-BRSP) and total Bradford-reactive soil proteins (T-BRSP) were found in weathered soft rock soils inoculated with G. mosseae and Rhizobium NWYC129 together. We conclude that dual inoculation with G. mosseae and Rhizobium NWYC129 was highly beneficial for improving the growth of black locust seedlings and the quality of weathered soft rock soils of the Loess Plateau.