The inventory of accurate area of farmland is one of the major tasks for land use investigation and detailed land investigation. The normal method is in situ measurement, and the work amount is enormous, with huge employment of manpower and high cost. This paper introduces a new effective method to measure and calculate farmland area by using aerial remote sensing technique. The method establishes regression equation of linear objects on air photos to revise the values that are directly measured on air photos for various patches of farmland with a surveying magnifier, and thus accurate farmland areas could be obtained. This method can. satisfy the demand for accuracy of detailed land invertigation with large scale air photos, save large amount of cost, and greatly elavate survey efficiency. The method allow indoor checking and revison for each patch of farmland.