This paper presents the Euclidean norm based new power quality index (PQI), which is directly related to the distortion power generated from nonlinear loads, to apply for a practical distribution power network by improving the performance of the previous PQI proposed by the authors. The proposed PQI is formed as a combination of two factors, which are the electrical load composition rate (LCR) and the Euclidean norm of total harmonic distortions (THDs) in measured voltage and current waveforms. The reduced multivariate polynomial (RMP) model with the one-shot training property is applied to estimate the LCR. Based on the proposed PQI, the harmonic pollution ranking, which indicates how much negative effect each nonlinear load has on the point of common coupling (PCC) with respect to distortion power, is determined. Its effectiveness and validity are verified by the experimental results from its prototype's implementation in a laboratory with a single-phase 3 kW photovoltaic (PV) grid-connected inverter, which contributes to a small distortion in voltage at the PCC, and practical nonlinear loads. Then, the harmonic current injection model based time-domain simulations are carried out to prove the effectiveness of the proposed PQI under the other conditions with different nonlinear loads.