Economical effect of Project of “Returning Farmland to Forest and Grassland” was related to stability of man-made ecological systems. It was very important to research the development condition of rural economy in areas of Grain for Green for scientific evaluation economical effect of the Project; A case study of Ansai County, a survey was carried out on change of rural economic effect after put in practice the Project though Participatory Rural Appraisal (PRA); The relationship between Economical effect of the Project and 6 economic indicators was analyzed by principal components analytic method; economical effects of 8 patterns were classified by cluster analytic method. The results of principal components analyzed indicate that 6 economical effect targets can simplify as the characteristic agriculture income factor, the grain income factor and the non-agricultural income factor, and it analyzes the detail of contribution of various factors to the economical effect. With the clustering analysis divided 8 patterns of take back from agriculture into 4 economical effect development types: synergize, level off, slow and detain. The research further confirms the relationship among the economical effect, the agricultural structure adjustment, the following industrial development and the surplus-labor shift, provides study basis for successfully development of the project.