The invention provides a flexible ring network controller capacity design method including the following steps: determining the rated capacity of an inverter side of a flexible ring network controller; determining the rated capacity of a rectifier side of the flexible ring network controller; determining the equivalent commutation reactance of the flexible ring network controller; and determining whether the capacity design of the flexible ring network controller is reasonable. According to the invention, the normal power transmission demand, the emergency active support demand under the condition of n-1 fault and the dynamic reactive power support demand are considered, the loss of the flexible ring network controller is also considered, and IGBT through-current capability check is performed. The design is comprehensive, the project is highly targeted, and the application and promotion value is very high. When a line fails in a sub-area grid, emergency active support can be offered to the sub-area grid based on quickly-controllable active power, which is of great significance to maintaining the safety and stability of the sub-area grid. The commutation reactance and the IGBT through-current capability can be optimized in a matching manner, and the technical efficiency is significant.