Current developments in agricultural aviation technology have gradually increased the requirements for the endurance of agricultural unmanned aerial vehicles (UAVs). It is significant to establish an endurance evaluation model for different types of UAVs and rationalize the battery and operating load parameters on this basis, which play an important role in improving the operational efficiency of the whole UAV. Taking the quad-rotor UAV as the research object, firstly, the structural composition of the whole UAV is studied in this paper, and then the main influencing parameters of the UAV endurance evaluation model are determined, combined with the mass distribution data of specific models. Based on the above, the hovering output power of the UAV is modularly divided, and a fast measurement method is innovatively proposed to sample and fit the power data of each module. Finally, in combination with the variation law of the lithium battery discharge time, a rapid evaluation method of the endurance time of agricultural UAVs is derived and validated by the hovering test data of the agri-drone with a standard operating load of 5 kg. The results show that the mass distribution system of the UAV has the highest percentage of operational load mass (40.45% under the standard configuration); the power distribution system has the highest percentage of motor pull curve output power (91.12% under the standard configuration); the maximum error of the surface verification point in the validation experiment is 2.73%, and the model is of relatively high accuracy. The modeling idea and the supporting module power rapid measurement method can quickly model and evaluate the endurance of common agricultural drones, guide reasonable matching between the battery and operating load, and are of great significance in improving UAVs’ endurance and operational efficiency.