Abstract: The identification of power construction project cost risk is a key factor of the effective control. There are many risks in the current power grid construction project cost control process, which cause various impacts on project cost. In order to control the cost of grid construction projects better, this paper established the risk system from the aspects of development risk and construction risk. This paper introduces the method of maximal tree algorithm and identifies the key risk factors of grid construction project cost by constructing a mathematical model and empirical analysis. The conclusion can be drawn that there are three types of risk factors in power grid construction project cost such as planning risk, research risk and cost risk, which provide effective decision making information for risk control.