Grassland is one of the largest terrestrial ecosystems in the world. The main factor limiting aboveground grassland productivity and diversity is topography, which can be maintained by enclosure. However, research on the impact of the interaction between enclosure and topography on grassland characteristics is scarce. This study was carried out on a typical steppe to explore the effect of enclosure duration and topography on vegetation characteristics. We examined aboveground biomass (AGB), species richness, and community coverage for three enclosure duration (0, 3, and 6 years), on four slopes (0, 15, 30, and 45 degrees), and two aspects (slope orientation, shady and sunny). The results indicated that: The AGB on the sunny slope of 15° and the slope of 0° had a higher value after 6 years of the enclosure. The AGB increased with the enclosure duration. Different trends were observed between shady and sunny slopes when the slope increased. The AGB on the shady slopes was higher than that on the sunny slopes. In addition, the species richness increased with the slope and the enclosure duration. The annual species richness declined as the slope and enclosure duration increased. All the biodiversity indexes increased with the enclosure duration. Margalef and Shannon-wiener indexes decreased with increasing slopes, while Simpson and Pielou indexes increased. Finally, the coverage decreased with increasing slopes and increased with the enclosure duration. This paper demonstrates that enclosure and aspect directly affect aboveground biomass, and slopes affects aboveground biomass by affecting other vegetation characteristics. In conclusion, moderate grassland management is beneficial to grassland productivity.