Artificial grassland establishment is one of the fastest and most effective ways to restore the productivity of degraded grasslands. Little is known about the effect of different types of artificial grassland establishment (i.e., single- and mixed-sowing grassland establishment with perennial grasses) on soil seed bank in degraded grassland ecosystems. Single-sowing population of a high yield species usually has a great standing biomass causing shading that may inhibit germination of seeds in soil seed bank. Thus, we hypothesized that there is higher species richness and seed density in the soil seed bank of single-sowing than mixed-sowing grasslands. Here, we investigated the soil seed bank in four-year old single-sowing and mixed-sowing and control (degraded) grasslands on the Qinghai-Tibetan Plateau. We found that the autumn seed bank of mixed-sowing grasslands had lower species richness and seed density than single-sowing grasslands, while the summer soil seed bank (persistent seed bank) showed little differences. There were differences in biomass among the three grasslands (single-sowing > mixed-sowing > control), but there was no differences in species richness of vegetation. In sum, our results of the autumn seed bank support our hypothesis. Greater above-ground biomass in single-sowing grasslands could generally cause low light availability preventing seeds in soil from germinating and support more seed output, which both may indirectly or directly result in the relatively higher species richness and seed density in the soil seed bank. Our results also suggest that artificial grasslands usually returning to native grasslands in terms of production and species composition after several years is likely due to stability of the persistent soil seed bank.