Distribution line loss is one of the important indicators of the economic operation of the power system, which embodies the planning, design and operation of the distribution network. In this paper, the concept of limit line loss of 10kV distribution network is perfected. The RMS method is used to establish the mathematical model of limit line loss of distribution network based on the load constraints at the inlet of the distribution network and the maximum load constraints at the load end. The genetic algorithm is used to solve the objective function. Analyze the interval and trend of the maximum and minimum limit line losses, compare the optimal power values of each load node under the condition of the minimum line loss, and analyze the rationality of the distribution network planning and the direction of improvement from the perspective of energy consumption. The research shows that the maximum and minimum calculation results of the limit line loss of the 10kV distribution network are in a reasonable range. The line loss of the distribution network under this topology structure is within this range, which provides a strong theoretical support for the power supply companies to issue comprehensive line loss rate indicators.