This paper proposes an agricultural farmland valuation instrument adopting cheap UP500 type GPS model and C8051F020 MCU of high performance-price ratio. In the algorithm, the optimized triangle algorithm is used to calculate area. The function of valuation according to area is designed based on this. Experiments and research of this design valuation instrument are taken. The experiment proves that this instrument can realize rapid and real-time measurement of farmland area under certain precision and calculating the total price