The invention discloses an online prevention and control risk decision-making method considering the protection and security automatic control cost, and belongs to the technical field of electric power system dispatching operation control. The online prevention and control risk decision-making method adopts a calculation strategy that online security and stability prevention and control optimization measure calculation considering operation behaviors of a protection device and a security automatic device is carried out preferentially on anticipated faults with high risk cost caused by actions, which are directly triggered by the anticipated faults, of the protection device and the security automatic device until the sum of the prevention and control cost and the control risk cost of the protection device and the security automatic device reaches a minimum value, so that the difficulty in weighing the calculation speed and the calculation precision of the dispatching operation personnel in setting an anticipated fault set is overcome, and the dispatching operation personnel can acquire online prevention and control optimization measures capable of effectively reducing the actual operating risk of a power grid by only setting anticipated faults as many as possible according to a descending order of the anticipated fault occurrence probability.