With the large-scale use of sensitive loads in oil fields, the voltage sag problem has become increasingly prominent. In recent years, oilfield distribution networks have promoted information construction, providing data support for the realization of voltage sag source segmentation. Based on the structural characteristics and load moment model of the mainline distribution network, the constraint relationship between the voltage monitoring value and the electrical distance of the line is established based on the similarity of the load moments. The information monitoring point suitable for the distribution characteristics of the mainline network load is constructed. Determine the method, and characterize the steady-state voltage distribution of the line by the linear relationship between the distance coefficient and the voltage loss, and use the relationship between the voltage increment and the electrical distance between the monitoring points to locate the voltage sag source segment. Simulation results demonstrate the effectiveness of the proposed method.