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With increasing use of power electronic devices in past decade, the produced harmonic pollution to the power supply is aggravating the power supply quality. The monitoring of health state of power network is therefore becoming an essential issue. Although conventional techniques like DFT or FFT are still widely applied in industry, some limitations arise particularly under supply-frequency drift and transient circumstances. The proposed iterative-loop approaching (ILA) algorithm can respond to the supply-frequency variation, the fundamental-amplitude/phase variation as well as the harmonics-frequency-amplitude/phase variation instantly and accurately. Furthermore, an abrupt change of fundamental frequency and amplitude can be also traced properly and promptly. The numerical examples supporting the performance of the proposed algorithm are presented.