In order to accurately identify and locate the main harmonic source, this paper proposes a method based on the active power harmonic contributions. Synthetically considering the influence of harmonic voltage and harmonic current, the method can quantitatively divide their harmonic responsibility and accurately determine the position of the main harmonic source. And it also overcomes the limitations in the main harmonic source localization by using the harmonic current or harmonic voltage contributions. Firstly, the calculation equation of the harmonic active power contributions on PCC which are from the system and the user side alone are deduced, then extended to the multi-harmonic source system. A general mathematical model is constructed, with which the position of the main harmonic source can be located theoretically. Finally, a simulation model with two nonlinear users as a research object is built and simulated in Matlab/Simulink. The simulation results are consistent with the theoretical analysis, and it verifies the correctness and validity of the method.