This research measured the relative intensity of inter-specific competition, competitive superiority and nitrogen resource separation in five grass-legume combinations: Leymus chinensis-Astragalus adsurgens; L. chinensis-Lespedeza daurica; L. chinensis-Medicago ruthenica; L. chinensis-M. sativa cv. Aohan; L. chinensis-Vicia amoena at four seeding ratios: 1∶0, 2∶1, 1∶2, 0∶1. Relative yield (RY), relative density (RD) and relative yield total (RYT) were used to determine competition performances and coexistence for different grass-legume mixtures. RYT of each combination was bigger than 1 at different times, indicating niche separation likely occurred between grass and legumes. Even though intra-specific competition intensity was bigger than inter-specific, A. adsurgens and M. sativa cv. Aohan had competitive superiority when in competition with L. chinensis. Conversely, L. chinensis showed competitive superiority when growing with L. daurica, M. ruthenica, or V. amoena. Defoliation significantly affected the competitive superiority of the dominant species. Reducing the proportion of high competitive species in seed mixtures could enhance the likelihood of coexistence. The RD used in this experiment is a better predictor than RY for community structure of grass-legume mixtures. The ability to maintain lower individual biomass with higher module density was one of the putative mechanisms for species coexistence in this study.