Harmonic pollution caused many negative effects on the power system. Harmonic contributions determination has become an urgent problem in the power quality management. Existing literatures on harmonic contributions determination mainly discuss the cases when the background harmonics remain constant, while the background harmonics fluctuate with time in the real power system. To achieve the accurate calculation of equivalent harmonic impedance in the case of background harmonics fluctuation, the subdivision complex maximum likelihood estimation method is proposed in the paper, by which harmonic contributions of the harmonic source of concern on the specific bus can be deduced. By dividing the background harmonic into change component and approximate unchanged component, the matrix expression of subdivision complex maximum likelihood estimation method is derived. A case study based on the IEEE 14-bus test system was conducted, which shows that the proposed method is valid for harmonic contributions determination in the case of the background harmonics fluctuation.