The hilly area of loess plateau is well known for its high erosion rate and sensitive to the global change for their vulnerable ecosystem. This paper simulated impact of climate change on net primary production in hilly area of Loess Plateau at Zhifanggou watershed by using vegetation-soil integrated model (VSIM). Based on analyzing the trends of climate for 50 years of study area, 4 climate change scenarios, which included 20% changes in daily precipitation, 2℃ increase chosen arbitrarily, synthesize 20% increase in precipitation and 2℃ increase in temperature were used for sensitive tests, and run VSIM under current climate condition and 4 climate scenarios during period of 1990—1999. The simulated results were compared with that of present climate to analyze the effect of possible changes in climate variables on the net primary production processes. The result showed that (1) Net primary production was more sensitive to temperature increasing than the change of precipitation, soil water condition was affect by precipitation significantly, and decreased while temperature increased. (2) Net primary production reduced dramatically for 2℃ increase in the average air temperature for all vegetation, and grasses and sub-shrubs behaved more sensitive than others. Among all vegetation types, net primary production of Form. H. rhamnoides changed significantly while precipitation changed, and Form. R. pseudoacacia, Form. M. pumila and Form. C. korshinskii changed slightly. (3) Synthesize 20% increased in precipitation and 2℃ increasesd in temperature, net primary production of the watershed still declined except Form. H. rhamnoides decreased less for its sensitivity to precipitation change. (4) Water use efficiency decreased while precipitation and temperature increased for most vegetation types except Form. H. rhamnoides and Form. A. gmelinii, and Water use efficiency increased for most vegetation types except Form. A. gmelinii.