In order to calculate the theoretical line loss of low-voltage distribution transformer district and improve the line loss management level by using automatic meter reading system, a Newton-Raphson method to calculate the theoretical line loss of low-voltage distribution transformer district by adopting the load electrical energy is proposed. On the basis of the equivalent resistance method and the electrical energy acquired from the watt-hour meters of customers, the mathematical model of regarding theoretical line loss of transformer district as a variable is created. The model introduces relationship between load shape coefficients and the average load current, and the line loss modified coefficient of unbalanced three-phase load. The procedure of the Newton-Raphson method to solve the model is presented. Case analyses show that the proposed method can not only calculate the theoretical line loss of low-voltage distribution transformer district, but also provide the information to identify unknown line loss.