To address the issues of inaccuracies in traditional return voltage polarization spectrum, specifically the inaccurate representation of return voltage at various charging times and the challenge in determining the central time constant precisely, this paper proposes a novel drawing method of the continuous return voltage polarization spectrum. Firstly, this paper analyzes the relationship between the return voltage curve and the equivalent circuit parameters of the insulation system. It then builds a continuous return voltage polarization spectrum model, extracting a more accurate central time constant. By linking this central time constant with the micro-water content of the transformer, the paper establishes an effective method for determining the transformer’s micro-water content. Finally, this paper analyzes continuous return voltage polarization spectra using six transformers in different aging states. The results show that this method accurately determines the central time constant, significantly enhancing the accuracy of micro-water content determination in transformers. This approach offers a new method to accurately assess the aging state of transformer oil-paper insulation.