Electric tractor is more environmentally friendly and more efficient than traditional tractor, which has become a hot spot in the field of Agricultural Engineering in China. In order to monitor the leakage phenomenon of lead-acid battery device, this paper takes the 25 horsepower electric tractor as the research object, takes the high-voltage monitoring leakage model of electric vehicle established by the overall insulation resistance model as reference, establishes the overall insulation resistance model of battery and DC brushless motor system to monitor the overall insulation resistance of battery and motor working system to ground. Firstly, the theoretical analysis is carried out to clarify the insulation resistance measurement principle. Secondly, the measurement results are observed by changing the insulation parameters and auxiliary measurement resistance value. Finally, the measurement principle diagram is built by using the Simulink simulation environment of MATLAB simulation software. The model is simulated by setting reasonable parameters. The measurement error is obtained by comparing the operation results with the theoretical values. According to the simulation results, the measurement error of asymmetric insulation resistance of motor side to ground is smaller than that of symmetrical insulation resistance of motor side to ground, and the smaller the auxiliary resistance value is, the smaller the measurement error is.