With the development of construction and increase of investment for the power grid in China, the cost control and evaluation has become increasingly prominent role. Because of the application of new technologies and the complexity and diversity of technical schemes, the traditional construction cost index for the transmission line could not fully and accurately reflect the projects’ cost level. Based on the kernel function in support vector machine (SVM) and principal component analysis (PCA), this paper presented a relative cost index for the line projects and built a evaluation index model that was solved by the Matlab 7.0. The validation result of data sample in the 220 kV transmission line project shows that the relative cost index of line project is more comprehensive and reasonable than the traditional index. The relative cost index is convenient to get the comparison, sorting and evaluation of the cost control level for the transmission line projects, which has some maneuverability and application value.