In this paper, the decentralized dynamic event-triggered load frequency control problem for multi-area power systems under switching cyber-attacks is investigated. A new switching cyber-attack model including Denial of Service (DoS) attacks and deception attacks occurring in the sensor-to-controller channel is considered, which is characterized by being composite in both the spatial and temporal domains. The attack model is characterized by compositing in the spatial and temporal domains. To reduce the network communication burden and mitigate the impact of switching cyber-attacks on the closed-loop system, we design a decentralized dynamic event-triggering mechanism (DETM) that dynamically adjusts threshold parameters using hyperbolic tangent functions related to output measurement deviations. Further, a decentralized secure LFC scheme is developed based on periodic sampling, which can reduce the consumption of communication resources under switching network attacks and protect the security and frequency stability of the power system. Finally, the effectiveness of the proposed method is verified by a simulation case of the IEEE 39 BUS test system containing 10 generating units.