As an important ecological project in China, returning farmland to forest is not only conducive to ecological protection, but also of great significance to enhance the function of forest carbon sink and achieve the goal of carbon neutrality. Evidences revealed that the conversion of cropland to forest has changed the carbon cycle process of the ecosystem by adjusting the way of land use, which could be quite helpful to strengthen the land's carbon sink capacity. Aiming at the carbon sequestration benefit of Grain for Green Project, this paper evaluates the potential carbon sink benefit of returning farmland to forests in Guizhou Province. On the basis of analyzing the cultivated land conditions and taking the objectives of Grain for Green Project into account, the areas that need to be converted are determined in accordance with the requirements of returning farmland, such as slope, rocky desertification degree and soil heavy metal pollution; The random forest classification algorithm is used with 9 environmental variables such as terrain, climate, soil and so on to discover the driving mechanism of vegetation growth, then the suitability of the cultivated land suitable for forest, grass and shrub is evaluated. According to the evaluation results, the carbon sink factors of the main vegetation types in Guizhou Province are modified to obtain the carbon sequestration rates of different vegetations, based on which the carbon sink potential of the Grain for Green Project is evaluated. The results show that:(1) there are 825250 hm² of steep slope cultivated land above 25° in Guizhou Province, 78813 hm² of 15-25° slope cultivated land in important water sources, 251400 hm² of severely rocky desertification cultivated land, and 93300 hm², 216600 hm² and 69200 hm² of the cultivated land seriously polluted by heavy metals Cd, Pb, Zn, respectively; (2) Within the 9 factors that has been considered when evaluating land suitability, rainfall, altitude and accumulated temperature more than 10℃ are three most important natural factors which have a great influence on the process of vegetation growth. Among 1319700 hm² of the cultivated land to be converted, the areas suitable for forest, grass and shrub are 635000 hm², 387100 hm² and 297600 hm², respectively; (3) The carbon sink potential of the new round of Grain for Green Project in Guizhou Province is considerably huge, after comparing with grassland and shrub, the contribution of forest land to carbon sink has always been the largest. The carbon sink increment of forest, shrub and grassland is 17790000 tons, 2930000 tons and 450000 tons, respectively, and the total increment of carbon sequestration can reach 21170000 tons, which will play a positive role in achieving the goal of carbon neutralization.