In this study, the grasslands sown with a single cultivar for the restoration of the "Black Beach", the severely degraded alpine meadow in the headwater areas of the Qinghai-Tibetan Plateau, were investigated. To determine when the plant community structure, composition, and diversity are close to those of the natural grassland and the threshold of the recovery time in terms of plant restoration succession, plant species composition, height, coverage, aboveground biomass, and diversity of both the plant community and functional groups of the cultivated grassland in different planting years were investigated. The objectives of this study were to establish a scientific basis for regulating restored grasslands through cultivation and provide theoretical guidance for the restoration of the degraded grassland in the Three Rivers Source region. The results showed that the number of gramineous plants increased significantly during the first five years after the establishment of the cultivated grassland, the height of plant communities increased by 847.6%, and the coverage plant communities increased by 134.5%. The diversity index of the grassland plant communities across the restoration years showed a growing trend, and plant community composition reached a steady state after eight years of grassland cultivation. After 16 to 18 years of grassland cultivation, the plant community reached a more stable state in terms of species composition and diversity. The Jaccard, Sorensen similarity, and Cody difference indices of the plant community between the cultivated grassland and the non-degraded natural grassland in 18 years of grassland cultivation were 0.596, 0.747, and 9.5, respectively. Consequently, the restoration of the "Black Beach" degraded grassland with grassland cultivation may generate a better recovery effect after 8 years of grass plantation, with appropriate regulation strategies to promote the progress of the cultivated grasslands towards natural conditions. The species composition of the cultivated grassland in 18 years was similar to that of the natural grassland, but with some differences. In the headwater areas, 18 years may be the minimum time needed to fully restore the "Black Beach" degraded grasslands into healthy natural grasslands through grassland cultivation.