Network topology optimization which is a new technology improves the distribution of system power flow and system safety and economy by changing the network topology dynamically during the power system operation. Aiming at the problems of poor flexibility and low computation efficiency in the existing static and dynamic network topology optimization methods, a semi-dynamic network topology optimization method is proposed in this paper. Based on this method, the auxiliary optimization of power system operation mode is carried out. Firstly, the advantages and disadvantages of current static and dynamic network topology optimization methods are described, and then a semi-dynamic network topology optimization method is proposed. Secondly, on the basis of static, dynamic, and semi-dynamic network topology optimization methods, a general form of auxiliary optimization model of day-ahead power system operation is constructed. By using linearization technology, the proposed dynamic economic dispatch model is transformed into a mixed integer linear programming problem. Finally, test results of the IEEE 14 bus system and IEEE 118 bus system demonstrate that the proposed method achieves the coordination both the power system operation flexibility and computation efficiency.