This paper investigates the problems of the residual-based actuator fault detection based on observer design for a class of uncertain linear systems. First, a reduced-order observer, which can estimate the original system states asymptotically when the system does not suffer from actuator faults, is designed. Second, an auxiliary system with both disturbances and actuator faults is constructed and its corresponding autonomous system is actually an asymptotical stable system. Third, for the auxiliary uncertain system, an interval observer, which is robust to disturbances but sensitive to actuator faults in a sense of interval estimation, is developed. Furthermore, based on the interval observer, a computation of the interval estimation of the output is given and then a residual-based actuator fault detection scheme is proposed. Finally, a simulation example is given to verify the effectiveness of the proposed methods.