The flat design of security and stability control system (SSCS) can realize the information interaction between each control sub-station and each sub-grid,which is conducive to reducing the number of SSCS layers and improving its reliability,thus increasing the difficulty of searching the control paths and differentiating the vulnerable components. The bidirectional/loop communication channels are added to the tree-shape SSCS to reduce the intermediate stations and find all the paths from the top to bottom stations based on the adjacent matrix. Considering availability of components and controllable capacity of underlying stations,the control capability of the flat SSCS is newly quantified. Average control ability and average associated path number are defined to evaluate the influence of flattening on the control ability of each station. Analytical sensitivity model of the control capability of flat SSCS with respect to the components' availability is proposed to determine the vulnerable components. The numerical analysis shows that the flat design reduces levels and intermediate stations,and improves the controllability of the SSCS. The impact on the controllability of the SSCS increases by the number of paths and controllable power,but decreases by the number of layers.