Grassland ecosystems play important roles in the global carbon cycle. The net primary productivity (NPP) of grassland ecosystems has become the hot spot of terrestrial ecosystems. To simulate the NPP in the grasslands of southern China, we built a land portfolio assessment (LPA) model. The LPA model was named according to the framework and principle of this model. From the framework of the model aspect, it was mainly driven by two parameters: leaf area index (LAI) and photosynthesis accumulation (PA). LAI is an extremely important structural characteristic of grassland and directly related to the exchange of energy, CO2 and mass at a variety of scales. PA is represented by the amount of net photosynthetic production based on fixed-point observation. From the principle of the model aspect, it is represented by the inherent implication of NPP and a part of land portfolio assessment. The results showed that the NPP values in the study area had a decreasing trend from east to west and south to north and that the mean NPP was 320 g C m−2 year−1 from 2001 to 2010. Correlations analysis showed that the correlation coefficient (r) between NPP and highest monthly mean temperature of a year was the maximum (0.6422), and the r value between NPP and annual precipitation was the minimum (0.3821). Using trial and error, the LPA model accurately simulated the NPP dynamics of southern China’s grassland ecosystem, and the results were biologically realistic.