With the expansion of smart grid, rapid development of new energy and extensive application of impact and nonlinear loads, the problems of power quality, such as harmonic, three-phase unbalance, voltage deviation, voltage fluctuation and flicker, are becoming more and more serious. The loss calculation of distribution network under the disturbance of composite power quality is the basis of analyzing the line loss composition, formulating the loss reduction measures and realizing the economic operation of power grid. At present, more than half of the power loss in distribution network is generated in the low-voltage distribution network. In this paper, the calculation model of loss of key equipment in low voltage distribution network under complex power quality disturbance is deduced theoretically, and it is simulated and verified by MATLAB/SIMULINK. On this basis, a decoupling method of complex power quality disturbance is proposed, which decouples the additional loss of the key equipment under the complex power quality disturbance, and the correctness of the decoupling method is verified by simulation. It lays the foundation for the establishment of the sensitivity mapping relationship between all kinds of power quality indexes and the additional loss of the power grid and the evaluation of the influence weight of each single power quality index on the additional loss.