A method for optimizing the flexible constraints of an electric power system includes a step S1 of expressing the total power generation cost of the electric power system by using the sum of quadratic functions of active power outputs of all generator sets in the system and constructing an objective function, a step S2 of selecting a multi-dimensional flexible optimization model or a flexible power generation cost optimization model according to the practical situation of the electric power system and the practical purpose of optimization, a step S3 of determining the operating conditions of the electric power system, and a step S4 of carrying out load flow calculation.