Large amount SO2 emission caused serious damage of forest ecosystem in China and calculation of the damage cost is an important issue for policy-making. However, no applicable method was developed to estimate forest damage under different SO2 emission scenarios. Basing on previous field researches on sulfur-related forest impact in China and recent critical load mapping research, this paper presented a model for forest damage calculation by developing a dose-response function that related the damage to cumulative sulfur critical loads. This model was applied to the forests in Hunan, a province in acid rain control zone in China. Results showed that in the business-as-usual case, SO2 emission in Hunan will increase by 120% from 1995 (8.82 mil. ton) to 2020 (19.56 mil. ton), but damage cost will increase by 4.3 times, reaching 6.19 billion RMB in 2020. Results also showed the measures for SO2 control were cost-effective because the marginal damage cost will be about 6000 RMB per ton SO2 in 2020 in BAU case. At current SO2 emission level, marginal benefit will be about 1500 RMB per ton. Uncertainty analysis demonstrated that this model provides reasonable damage estimates and would therefore be applicable in a broad range of policy settings.