Abstract: Agricultural machinery played an important role in the processes of farming, sowing, and harvesting, closely related to national food production security. Accurately calculating the field operation area of agricultural machinery was a critical basis for subsidy distribution, fee collection, efficiency evaluation, and income calculation. Precise and effective operation area calculation was an urgent need for agricultural machinery management departments, agricultural machinery manufacturing enterprises, cooperative managers, and operators. This study addressed the demand for precise calculation of the effective operation area of agricultural machinery and proposed a method for calculating the operation area of wheat harvesters based on spatiotemporal trajectories and grid key points. It considered the positional relationship between boundary grid key points and trajectory vector rectangles, calculating the mathematical expectation of various occupancy scenarios to effectively remove overlapping operation areas while improving calculation accuracy. In response to the failure of the grid buffer zone method to fully analyze boundary grids, this research introduced a grid key point algorithm. The algorithm analyzed possible situations at boundary grids based on the positional relationship between grid vertices and center points with vector rectangles, computing mathematical expectation values for each scenario and incorporating steps for area calculation. The study obtained trajectory data of combined harvesters during the wheat harvest period from the national agricultural machinery big data platform, including fields such as latitude and longitude, time, vehicle ID, speed, and direction. The selected trajectory data had meter-level positioning accuracy and a sampling frequency of 0.2 Hz. After data cleaning and segmentation of fields and roads, the study chose trajectory data from 50 farmland plots in major wheat-producing regions (Hebei, Henan, Shandong, etc.) for the years 2022 and 2023 as the test dataset. A sensitivity analysis of the bottom grid edge length was conducted. Considering the accuracy of area calculation and algorithm execution speed, the study ultimately determined the bottom grid edge length for both the grid buffer zone method and the grid key point method to be one meter. The study queried the operational width of the selected combine harvesters and calculated the area of the 50 farmland plots using three methods: distance swath method, grid buffer zone method, and grid key point method, comparing them with true values to calculate the average calculation error of the three methods. Results showed that the grid key point method had an experimental average calculation error of 2.66%, which was 69.45% lower than the distance swath method and 1.25% lower than the grid buffer zone method. The grid key point method could solve the problem of overlapping operation areas in farmlands and exhibited higher calculation accuracy compared to similar algorithms. Additionally, the proposed algorithm in this study was based on agricultural machinery operation trajectory data, meaning that whether it was rice harvesting, corn harvesting, or wheat sowing, only operation trajectory data and operation width needed to be provided to complete the operation area calculation, showing a certain degree of versatility. In conclusion, the algorithm proposed in this study effectively improved the accuracy of operation area calculation and could provide a basis for agricultural subsidies and policy support, cost control and budget planning, and assessment of agricultural production efficiency.