To address pasture degradation on the Tibetan Plateau, the Chinese government has launched the ecological restoration project Grazing Withdrawal Program (GWP) since 2004. However, few studies have evaluated the impact of the GWP on grassland recovery. Based on monthly remote-sensed vegetation index and meteorological data from 2000 to 2012, we assessed the dynamics of annual net primary productivity (NPP) in alpine grasslands and quantified the effects of climatic factors and anthropogenic activities on NPP change by using the climate-driven NPP and the Carnegie-Ames-Stanford Approach (CASA) models. We found that there existed two distinct periods with an accelerating trend in NPP increase before and after 2004. The area percentage of NPP change induced by climatic factors increased from 41.55% to 83.75%, but that percentage caused by human activities decreased from 58.45% to 16.25% in the two periods of 2000–2004 and 2004–2012. Between 2000 and 2004, overgrazing reduced the positive effect of climate change on NPP variability, resulting in wide-scale grassland degradation. Between 2004 and 2012, grassland ecosystems gradually recovered from heavy grazing pressure, and the human induced degradation was reversed after the implementation of the GWP. Thus, temperature and solar radiation became dominant factors in driving NPP change. Our results indicated that the GWP produces a significant positive effect on the restoration of alpine grasslands by controlling livestock numbers and decreasing grazing intensity. This study provides an objective assessment of restoration actuation on grassland ecosystems, having important implications for demonstrating the effectiveness of the GWP on grassland restoration on the Tibetan Plateau.