Aim at the lack of area calculate method based on Global Positioning System (GPS) in researching of precision agriculture, a new area calculate method was put forward, which is called triangle division method. The method received area borderline orientation data by GPS, then made area polygon by coordinate conversion. The total areas which want to be measured can be calculated by analyzing these triangle areas. As an example, the area of a piece of field of 65m×58m was calculated by receiving data through GPS by using this new method. The new method was used to measure and calculate. The result showed that this method can calculate polygon area very quickly with high precision. It can be used to measure, orientation and layout based on GPS.