A novel real-time line loss calculation method based on state characterization is proposed for calculating the line loss of distribution network. First, the load currents collected by the measuring terminals during a certain period are characterized, such that the change characteristics of electric loads are reflected fully, and the pressure of data transmission and data processing is relieved effectively. Secondly, the characteristic current in the low voltage side of the pubic distribution transformer is decomposed symmetrically by the sequence component method, then the characteristic current in the other side can be calculated according to the connection way of transformer. Thirdly, based on the state characterizations, the line loss of distribution network is calculated by the split phase calculation method, solving the problems about the three-phase unbalance and the accessing of distributed power generation. Finally, the proposed method is applied to a real distribution network, and the results prove its effectiveness. This work is supported by China Southern Power Grid Co., Ltd. Science and Technology Program (No. GDKJ00000049).