Power flow data generation based on similar day selection is proposed to improve the accuracy and fineness of security check for scheduling plans. Integrated similarity degree is defined to select basic operation mode from historical operation modes of similar days. Topology information, reactive data and generator voltage data are extracted from basic operation mode. Bus voltage can be kept near the voltage value in the basic operation mode through auto-switching of reactors and capacitors during power flow calculation to form converged and reasonable internal network power flow data. Operation mode of external network with similar boundary nodes is selected with largest integrated similarity degree. The whole power flow data is generated through the joint of external and internal network power flow data. External network is adjusted to maintain the boundary node voltage and tie line power as the value of internal network. Real engineering applications in actual power grid have fully confirmed the practicality and validity of this method.