As one of the key infrastructure of power industry, the network security of power monitoring system is of great significance to the stable operation of power grid. At present, power monitoring network security protection technology can not guarantee that all network attacks can be effectively protected. Based on this, a new active detection and protection strategy is proposed to solve the problem that the power monitoring network can not effectively detect the multiple types of network attacks. Based on D-S evidence theory, the deviation degree of security incidents is calculated. According to the deviation degree, the local linear weighting is used to determine the weight of safety incidents, and the evidence combination of safety incidents based on local linear weighting is formed. On this basis, for multiple types of network attacks, a composite attack detection and protection strategy based on evidence classification is constructed. The experimental results show that the scheme not only has a very good discrimination accuracy for a single attack source, but also can effectively identify multiple types of network attacks, which has reference significance for the safe operation of power grid.