In view of the low automation level of distribution network, the changes of node electrical data and topology structure cannot be fully monitored in real time, which makes the line loss calculation difficult. This paper proposes an online calculation method of distribution network line loss based on random forest （RF） and kernel ridge regression （KRR）. Firstly, the typical operation modes under different topologies are determined according to the historical operation conditions of the distribution network. Secondly, by the power flow calculation of distribution systems with different operation modes, the training sets are constructed respectively. Through the training of KRR model, the mapping relationship between branch line loss power and distribution network state is fully explored. Finally, the operation mode of distribution network is judged based on the RF classification method, then the online calculation of distribution network line loss is realized by the KRR model of the corresponding operation mode. The example analysis results of IEEE 33-bus distribution network show that this method has higher accuracy and better robustness.