Condition evaluation of GIS is an important approach to providing decision-making for condition maintenance. Extension analysis is introduced into condition evaluation of GIS. With the basis of indexes of SF6 gas components, combining with the purity, air humidity and air content of SF6 indexes to construct correlation functions according to SF6 gas components are widely used to the operation of GIS. Determine the relationship between the condition level and the corresponding maintenance strategies based on relations of correlation function values and note values and warning values specified by procedures. Entropy method is used to solve the weight problems of each index for GIS condition assessment. The results of an example analysis indicate that the method in this paper is capable of assessing the state of GIS in each chamber and it is conducive to the monitoring and early warning and developing comprehensive maintenance strategy.