The invention provides a cascading failure risk calculating method for a large power grid. The cascading failure risk calculating method includes steps of 110, initializing the power grid, forming an initial network with certain network nodes and corresponding power transmission lines according to a certain mode; 120, calculating network cascading failures; 130, calculating risk value of the network cascading failures by formula according to the calculating results of the step 120; 140, increasing m network nodes for the initial network and repeating the step 110. By the cascading failure risk calculating method, calculation can be performed for different power-grid increase modes, and risk in the power grid increase course in different increase modes can be judged, so that different construction modes and network scales can be selected according to different conditions, and power transmission grid structures and planning schemes in proper scale and with reasonable power source and load point distribution can be selected, the power grid structure and transmission capability can be reinforced by constructing power source support points in a load concentration center, and risk of the cascading failure is reduced.