Mizhi County,as the part of the Loess Plateau of Northern Shaanxi Province,nowadays,its environmental conditions are not suitable for economy development and environment construction.So,it is essential to go in for further study of converting farmland to forest and grassland and eco-environment construction in the county.The paper aims to assess the achievements of converting farmland to forest and grassland and accelerate the course of eco-environment construction in the underdevelopment area.Some conclusions are drawn as follows.(1) Taking the program testing information in the county from 1998 to 2003 as a study object,and taking the social,economical and ecological situations before the program testing as contrasts,the paper gives an objective evaluation to comprehensive benefits in the county.Results show that the benefits of society,economy and ecology are all enhanced effectively.(2) Converting farmland to forest and grassland has significant influences on population distribution and landuse change.Among population and land resources,the total of agriculture labor,outworking labor and arable land change rapidly.(3) Influenced by converting farmland to forest and grass land,economical benefits in the county appear increasing step by step.General domestic product and forest product will grow notably along with the applying time of converting farmland to forest and grassland.Rural economics also increases significantly.(4) Rural economic growth is the most rapid among all of industries in the county.(5) Eco-environment benefits embody in soil and water conservation effectiveness.(6) The long time protecting mechanism must be set up in order to enforce the achievements of converting farmland to forest and grassland in the county.