Aiming at problem of line loss in a three-phase four-wire low voltage distribution network, this paper presents a method for calculating line loss under the action of composite power quality and compares it with that under the action of single power quality. First, the line loss caused by three-phase imbalance is deduced. Secondly, the line loss under harmonic condition is derived according to the superposition principle. Then the line losses under the combined action of the two power qualities are comprehensively considered and derived, and the line losses under the combined action of the two power qualities are compared by the symmetric component method. The results show that the three-phase imbalance aggravates the harmonic loss, that is, the line loss when the harmonic and the three phase unbalance exist together is greater than the sum of the line loss when they exist separately. Finally, the Matlab/Simulink simulation tool and chroma 6590 programmable AC source experiment are used to test each part and verify the correctness of the theoretical derivation. This work is supported by National Natural Science Foundation of China (No. 51877141) and Science and Technology Project of China Southern Power Grid Company (No. 0002200000042389).