Seasonal changes in grassland vegetation were studied in Northern China. Grazed areas had lower vegetation cover, species richness, and biomass in April compared to fenced areas, while plant density showed the opposite trend. Fenced areas had higher vegetation cover and biomass in June and August. In grazed areas, the relationship between plant density and biomass changed from positive in April to negative in August. Dominant species showed varying relationships between plant density and biomass. Long-term grazing and seasonal changes significantly influenced vegetation cover, plant density, and biomass in the grassland. Grazing also affected the seasonal changes in plant density and biomass of dominant species, impacting the relationship between plant density and biomass.