The East Asian summer monsoon has a significant impact on climate change in the Chinese monsoon region, but the responses and mechanisms of the terrestrial net primary productivity (NPP) to summer monsoon climate change are still not clear. In this paper, an atmosphere-vegetation interaction model (AVIM2) is used to simulate the NPP in the Chinese monsoon region. The correlation between NPP and summer monsoon index is analyzed, and the response mechanisms of NPP to the variations of summer monsoon are discussed. It is found that the vegetation responses and mechanisms to the variations of the summer monsoon are very different in North China and South China. Corresponding to strong summer monsoon years, the NPP in North China increases, while the NPP in South China decreases. The East Asian summer monsoon can affect precipitation over the North China Plain during vegetation growing seasons, and thus also affects the local NPP. Due to the superimposed effect of temperature and precipitation on NPP, Beijing, Tianjin, and Hebei become the most sensitive areas to the variations of the summer monsoon in the North China Plain region. During strong East Asian summer monsoon years, the NPP in South China decreases, and the main impact factors correlated to a strong East Asian summer monsoon are different: In Jiangsu, Anhui, Hubei, Hunan, and Jiangxi provinces, it is the reduced solar radiation; in Guangdong and Taiwan provinces, it is the lower temperature; and in Zhejiang and Fujian provinces, the major factor is the superimposed effect of reduced solar radiation and lower temperature.