The impacts of climate change on net primary productivity (NPP) and carbon budget (NEP) of forest ecosystems in Northeast China (38°43′N～53°34′N, 115°37′E～135°5′E) were simulated under present climate condition and different climate scenarios using forest carbon budget model FORCCHN that based on growth process of individual trees. The results showed that: from 1981 to 2002, the total amount of NPP in study area fluctuated between 0.27 and 0.40 PgC•a-1, and the mean value was 0.34 PgC•a-1. The total soil respiration was from 0.11 to 0.27 PgC•a-1，and the mean value was 0.19 PgC•a-1.The total amount of NEP was from 0.11 to 0.18 PgC•a-1; The forest played a positive role in absorbing carbon dioxide during past twenty years, and every year it absorbed 0.15 Pg C carbon dioxide on average. NPP and NEP were more sensitive to increase of air temperature than the change of precipitation; Synthesizing 20% increase in precipitation and 3℃increase in temperature, NPP and NEP enhanced rapidly, and finally minimal changes were found under scenarios of no change in air temperature and increase or no change in precipitation.