Aiming at the problem that oil-paper insulation aging evaluation accuracy is vulnerable to single characteristic parameter under different circumstance, a new comprehensive gray evaluation method for transformer oil-paper insulation status based on dimension reduction techniques and K-means clustering is proposed. First, a terminal two-point analytical method of depolarization current spectrum is presented to extract three new features of insulation aging. Second, a multivariate evaluation index array is constructed and the comprehensive index Z is refined via a combined weighting method combining with main component dimension reduction techniques. Finally, a hierarchical database of ‘good, middle, and poor’ 3-level for Z is built by weighted Euclidean distance-based K-means clustering algorithm, the level limit standard table for comprehensive index is determined and the grey clustering evaluation system for transformer oil-paper insulation aging state is set up accordingly. The final numerous case analysis proves that the proposed method is comprehensive and accurate to evaluate the insulation state. This work is supported by National Natural Science Foundation of China (No. 61174117).