Abstract: The paper introduced traditional calculation methods of theoretical line loss of distribution network, and pointed out that electricity quantity method and root-mean-square method would cause too small theoretical energy loss and too large management line loss because of insufficient consideration of change of load curve. For above problems, it put forward a calculation scheme for theoretical line loss of distribution network by use of improved Newton method, which does power flow calculation according to power records of 24 h of each substation in the whole distribution network, so as to obtain total line loss of the distribution network. It used electricity quantity method, root-mean-square method and the improved Newton method to calculate theoretical line loss of a 10 kV distribution network separately, and got a conclusion that results calculated by the improved Newton method are closer to statistical value than the other methods.