To improve insulation diagnosis accuracy of XLPE cables, the idea of comprehensive insulation evaluation based on multiple parameters is proposed. A evidence fusion diagnosis model based on fuzzy theory is established which uses fuzzy set and membership function to construct the distribution of reliability function of evidence theory and effectively solve the difficulty of determining reliability allocation of each evidence, on the basis of the model, evidence theory combines the credibility of each evidence to form the final insulation assessment result. Example analysis shows that the proposed model can effectively estimate the insulation of XLPE cables, and has a certain theoretical and practical value.