To assure the construction and operation security of the smart grid, it is necessary to assess the real-time operation reliability of the power cables which work in power tunnels by using on-line monitoring data. An online assessment system, which presents the important operating characteristics of the power cables, has been built up based on the following data: some preventive experiments results data, working environment, operation records, maintenance records, etc. With analytic hierarchy process method, an improved fuzzy integrated evaluation assessment model for cable assessment is proposed, the relative interiority indicator is adopted, and different methods are used to determine the membership functions for the qualitative indices and the quantitative indices. Through assessing the layer and sub-layer factors, the comprehensive operating conditions of the cables can be estimated. Based on the actual monitor data for a 220kV cable in power tunnel, the analysis result shows the proposed assessment model is correct and effective.