Because of the imperfect planning of low-voltage distribution network construction in the early stage and the line modification for balanced load distribution, the recorded information of the topological relationship between transformers and users is inconsistent with the reality. It is difficult to calculate the theoretical line loss accurately in distribution transformer area. In this paper, a method of calculating theoretical line loss based on equivalent resistance of distribution network is proposed. Using the real-time voltage and current data of transformers and users, the equations of resistance parameters of low-voltage distribution lines are established. The least square method is used to solve the problem, and then the theoretical line loss of the transformer area is solved by using the measured data. The method proposed in this paper does not depend on the distribution network topology, and can calculate the theoretical line loss range of low-voltage transformer area according to the measured data.