The systematic， synthetic and sequential preventing measures are one of the important methods of avoiding large-scale blackout. Most of the currently applied forewarning and preventive control systems focus on the dynamic， on-line and real-time security and stability analysis and precautions， but it is considered in this paper that proceeding from the whole process of power grid operation and production will better realize comprehensive prevention. The system engineering concept is applied to four important steps in power grid operation and production for direct security and stability analysis and preventive control. The crux of this method is security and stability analysis and the preventive control measures for four time step-up grades， i.e. weekly planning， daily planning， pre-operation and real time planning. The implementation and key techniques of a four-grade forewarning and preventive control system for secure and reliable operation of Henan power grid is described.