The measurement method of irregular field area is studied in this paper. A multifunctional measuring instrument of farmland area is developed by using GPS model. The instrument mainly consists of a GPS module, a micro-controller, keyboards, a LCD and a micro-printer. It can measure the area of irregular field rapidly, set the unit price charge of agriculture mechanical operation and calculate the total price, save and print the work record. The way to measure irregular field area, and how to display, store and print the measurement results are presented. The experiment shows the relative error of this instrument is less than 3%, and the larger of the area, the smaller the relative error is. The rapid and real-time measurement of field area is realized and the requirement of automatic valuation for agricultural machinery according to area can be satisfied by using this instrument instead of conventional measurement of using tape