A suitable ecological-water requirement of vegetation model was developed based on field observations in the karst peak-cluster depression area in Tiandong County of Guangxi Province. The ecological-water requirement and ecological water deficiency of vegetation in different seasons were estimated, and the main driving factors were analyzed. The results showed that the greatest ecological-water requirement of 4 dominant vegetation was in August, and the least was in January. The ecological-water requirement of Miscanthus floridulus is the greatest; the monthly average maximum, optimal, and minimum water requirements for this species were 36.85 mm, 25.80 mm, and 23.95 mm, respectively. The ecological-water requirement of Cipadessa cinerascens is the least; the average maximum, optimal, and minimum water requirements for this species were 23.99 mm, 16.80 mm, and 15.60 mm, respectively. The pattern of ecological-water deficiency is the same with the ecological-water requirement. The ecological-water requirement of vegetation has a significant positive correlation with vegetation coverage, leaf area index, air temperature, and soil bulk density; however, there is a significant negative correlation between ecological-water requirement and field water holding capacity. Vegetation coverage, soil bulk density, soil moisture content, and potential evapotranspiration are the main driving factors affecting the ecological-water requirement of vegetation.