A method for fault detection (FD) of networked control systems with unknown network-induced delay and unknown input based on eigendecomposition, adaptive evaluation and adaptive threshold is proposed in this paper. In the method, a known structure matrix of network-induced delay which is supposed to be smaller than one sampling period is extracted by eigendecomposition and an observer based residual generator is constructed. Then an adaptive evaluation function is proposed to ensure the optimal robustness to network-induced delay and unknown input at the evaluation stage based on the Parity Space approach. Finally, based on the estimation of the maximal influence of unknown random delay and unknown input on the evaluation function, adaptive threshold is designed to enhance robustness of the FD system and reduce the miss detection rate as much as possible.