The degradation land of rocky desertification is one of the most serious problems on economic development, ecosystem restoration and reconstruction in Southwest China. Grassland is one of the main vegetation types in karst area, and its fine root is sensitive to the response of environmental conditions, which determines the growth and development of plants and soil nutrition status. Fine root also has an indicator effect on the environmental changes, and can reflect the health status of grassland ecosystem. As the intermediary connecting the aboveground and underground parts of vegetation, fine root plays an important role in the matter exchange and energy transfer between plants and soil, helps plants sequestrate carbon and provide for nutrient and water uptake into plants. Fine root decomposition and turnover contribute significantly to element cycling in grassland ecosystems. In karst area, fine root dynamics are controlled by the complex interaction of environmental and biotic factors. In order to explore the contribution of fine root to soil nutrients in three grasslands with different degrees of rocky desertification, the sequential soil coring and root bag methods were used to study the fine root biomass, dynamic of decomposition and release of nutrient and their responses to rocky desertification in the potential, medium and severe rocky desertification grasslands from March 2017 to January 2018. The results showed that the fine root biomass increased first and then decreased with the seasons in the three types of grasslands with different degrees of rocky desertification, while decreased with the intensification of rocky desertification. The fine root biomass was 3355.65 g/m², 2944.02 g/m² and 1806.80 g/m² in the potential, medium and severe rocky desertification grasslands. The decomposition rate of fine root showed the "fast-slow" trend, and the residual rate of dry matter was all less than 50% after 300 days. The releasing process of fine root organic carbon, nitrogen, phosphorus and potassium was significantly different, and the pattern of releasing was "release" finally. The annual return amount of fine root organic carbon, total nitrogen, total phosphorus and total potassium was 32.46-161.08 g/m², 0.24-3.88 g/m², 0.08-0.32 g/m² and 0.15-2.78 g/m², respectively. With the intensification of rocky desertification, fine root biomass and fine root decomposition rate showed a gradually decreasing trend, while the releasing of soil organic carbon and total nitrogen showed a gradually increasing trend. Therefore, knowledge of fine root dynamics and nutrient release across a broad karst area with the ability to examine the feedbacks occurring between fine root and rocky desertification is critical to our understanding of the nutrient cycle and for protecting degraded grassland and restoring vegetation.