The invention provides an ultrathin 240-grade modified polyesterimide enameled rectangular copper wire which can overcome the defects of low space utilization factor and small specific surface of a common polyesterimide enameled round copper wire, and also effectively solves the problem that common polyimide enamel is not suitable for being transported and stored by production enterprises under normal temperature, thereby being suitable for the production of enameled rectangular copper wires. The ultrathin 240-grade modified polyesterimide enameled rectangular copper wire comprises a copper conductor of which the periphery is coated with a polyesterimide enameled layer. The copper conductor is characterized by having a rectangular section of which the wide side is 1.2-14 mm in size and the narrow side is 0.2-1.00 mm in size.