By taking ALOS as remote sensing information source and using supervised classification and human-computer interactive interpretation,the actual area of returning cropland to forestland project is extracted.Since complete measurement for the whole area is expensive and time-consuming,the map-spot area in remote sensing interpretation is selected as auxiliary sampling factor by using sampling methods of probability proportional to size(PPS).The total area of the project is estimated according to the areas of 30 engineering projects and the areas measured by GPS.The areas extracted from GPS and RS are then compared and the precision is up to 86.2%.This study is of importance in controlling the progress of project and its quality.