The basic farmland within the protection areas was required to be of high-quality and connective in the general plans for land use. A new method was developed to delineate the boundaries of high-quality, concentrated and connective cultivated land. Based on mathematical morphology principles and GIS methods, the high quality of cultivated land blocks could be identified through dilation and erosion operations according to this rule that which inside distance was less than threshold d1 and the number of blocks was larger than 3. This method was validated with the farmland classification data of Miyun County and Pinggu District in Beijing City, and 91% of high-quality, concentrated and the connective cultivated land could be identified by this method, so it would provide the reference method for delineating the basic farmland areas scientifically and reasonably in the general plans for land use.