With the increase of distributed generations, the problems related to lack of inertia and damping support for cascaded-type power networks are becoming more and more prominent. This may lead to oscillation and even system instability. To address this problem, a decentralized control with adaptive virtual inertia and damping combination method is proposed for islanded cascaded-type virtual synchronous generator (VSG) systems. Firstly, a model of islanded cascaded-type VSGs was developed to analyze the impact of changes in inertia and damping coefficients on frequency characteristics during dynamic processes. Subsequently, a decentralized control was proposed, which employs an adaptive method for regulating inertia and damping coefficients in real-time. The scheme effectively suppresses power and frequency oscillation, leading to improved dynamic frequency performance. Furthermore, this decentralized strategy only requires the local voltage and current measurements, ensuring a fully decentralized implementation. The cascaded-type VSGs demonstrate satisfactory reliability. The stability of the proposed strategy has been demonstrated, and guidelines for designing key control coefficients have been validated through the Lyapunov energy function method. Simulation and experimental results validate the proposed method.