Unmanned helicopter is becoming more and more popular in the field of agricultural plant protection, with the development of agricultural mechanization and modernization. And flight control technology is the core of unmanned helicopter. In view of the irregular shape of farmland, designing the flight route automatically and efficiently is one of the key technologies, which make sure the application of unmanned helicopter in the field of agriculture and improvement of commercial efficiency. With technical requirements of unmanned helicopter flight trajectory in agricultural plant field, the author of this paper researched irregular farmland identification and the optimal route design algorithm using the theory of graphics, and realized the corresponding function module and application software. The software has been proved in the engineering application, and achieved good application effect.