In this paper, the delta operator is applied to describe the networked control system with time-varying delay and random packet loss. A robust fault detection filter based on the observer is proposed to generate the residual signal, thus making the residual systems to be asymptotically stable and satisfy the H-infinity performance in delta domain. The parameters of the designed fault detection filter are obtained by linear matrix inequalities(LMIs). The numerical example is provided to verify that the proposed filter can effectively detect faults.