Analysis of power grid operation mode is essential for power system security. Due to the effects of electricity market and energy-conservation power generation dispatching， the power flow mode will change a lot comparing with the equal-usage power generation mode. The latter method for power grid operation modes is too simple to ensure the comprehensiveness of security assessment. A new classification method is proposed. The characteristic variables are extracted by combining cluster analysis and graph theory， and then used to establish the security constraint sets of each class. The proposed new power grid operation mode elaborative management method is presented and its effectiveness is verified by a practical example.