Forage quality is a key property of grassland ecosystems. In this study, grassland forage qualities were measured at 373 sampling sites throughout Guizhou Province in the karst mountain region of Southwest China, and the factors affecting it were explored. The forage quality level of most plant species was categorized into four levels: (1) preferred forage species; (2) desirable forage species; (3) consumed but undesirable forage species; and (4) non-consumable or toxic forage species. High temperature and precipitation appeared to facilitate the growth of preferred forage species, but limited the growth of other plants. Increasing soil pH had a positive impact on the number and biomass of preferred forage plants, but a negative influence on other plants, especially non-consumable or toxic plants. Both GDP and population density had a positive correlation with the number and biomass of preferred forage species, while such correlations for other levels of forage species tended to be negative. Grazing could lead to a decrease in the preferred forage species. Therefore, it is suggested that by focusing on soil improvement in grassland and maintaining an appropriate grazing intensity, global warming and rapid economic growth in Guizhou Province will likely contribute to increase the forage quality of karst grasslands in Southwest China.