The photosynthetic characteristics of three kinds of leguminous forages were studied in a karst rocky desertification area of Guizhou using the photosynthetic apparatus on sunny days. The results showed that the curves of annual diurnal variation of the net photosynthetic rate and transpiration rate on three kinds of experiment forges showed different variation trends that were mainly related to the experimental materials. They showed that there were significant differences in the net photosynthetic and transpiration rates of different grasses, and also in photosynthetic physiology changes of different seasons. Through the annual photosynthetic physiological studies among Trifolium repens, Trifolium pretense, Medicago sativa, and other forage legumes can be concluded that the highest annual average net photosynthetic rate of leguminous forage was Medicago sativa followed by Trifolium repens, and Trifolium pretense was the lowest; the highest transpiration rate was Trifolium pretense followed by Trifolium repens, and the lowest was Medicago sativa, with little differences of this three leguminous forage. The highest photosynthetic efficiency and the lowest transpiration rate make the water use efficiency of Medicago sativa the highest, followed by Trifolium repens, and the lowest was Trifolium pretense. In most months, Trifolium repens, Trifolium pretense, and Medicago sativa were sensitive to high photosynthetic active radiation and high temperature, so taking a dormant form with closing their pores to reduce its water loss and material consumption. The primary productivity of grassland depends largely on water use efficiency. When establishing artificial grassland in the karst rocky desertification area we need to consider the net photosynthetic rate, transpiration rate, and water use efficiency, and take measures to use different kinds of forage, which not only reduce water loss but also improve water use efficiency and ease drought and water shortage in rocky desertification areas. From considering the overall perspective of annual photosynthetic physiological characteristics, Medicago sativa is more suitable than Trifolium repens and Trifolium pretense for the habitat conditions in light-moderate karst rocky desertification areas.