In order to meet the requirements of cost management of power transmission and transformation projects and look for the deep level factors influencing the cost, the multiple regression method is used to find the scale factors influencing the cost of power transmission and transformation projects, based on these scale factors, the projects are classified reasonably, then the direct contrast among projects of different scales can be realized. Through the analysis of influencing factors, the scale factor, market factor, external environmental factor, the design techniques and construction standards are distinguished, and the key factors of the cost reasonability evaluation are also determined. Through building the influencing factor index and total index, the regional cost index system is built to provide criteria for cost reasonability evaluation. By building the cost reasonability evaluation index system, the costs are quantitatively evaluated from three aspects including regions comparison, annual cost comparison and concentration degree. Some research results of this paper have been applied to the 2015 cost analysis report of China Southern Power Grid.