It is a widely recognized one of the most cost-effective way to increase forest carbon sinks to control the rising of CO_2 concentration in atmosphere through afforestation and reforestation. Carbon sinks of artificial plantations are also recognized as a probable and likely way to mitigate global climate change. According to the national and international calculation rules of tree-planting CO_2 emission reduction,this study proposed a method to calculate the effect of the large-scale tree-planting CO_2 emission reduction through afforestation and reforestation based on the characteristics of the natural environment in arid areas. This paper discussed the ways how the calculation factors of the emission reduction were selected and suggested that a comprehensive research system should be put in place to reinforce the researches on forest carbon sinks from the perspectives of policy,technology and marketing,which could clearly and quantitively measure our country s contribution to mitigate global climate changes through afforestation and reforestation in the arid areas. The results of the study provided useful support to the development and the management of afforestation carbon sequestration projects in arid areas.