There are a lot of quantitative performance characterisation methods for composite insulators. However, how to apply the characterisation methods with different dimensions to comprehensively assess the health state of composite insulators remains a big problem for operators and researchers. A comprehensive health assessment model for composite insulators based on the Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS) method is proposed in this study. Fourteen composite insulators from four regions in Yunnan Province, China, and five quantitative characterisation indicators of the insulators, namely hardness, hydrophobicity, loss of hydrophobicity, recovery of hydrophobicity, and trap charge, are selected to verify the model. The results show that the Pearson correlation coefficient of the TOPSIS-based comprehensive health score with operating time is 0.72, which is better than the other single indicator. By introducing the operating time into the TOPSIS-based model, we can also obtain the ageing resistance performance of the composite insulator. This work provides a new idea to assess the health state and ageing performance of composite insulators.