Fault detection for linear switched systems is studied by using the H_/H_∞ unknown input observers method. Unknown input disturbance can be divided into two parts. Unknown input which can be decoupled is eliminated from the estimation error, and weighted H_/H_∞ mixed performance index is used for representing the effect of fault and the remaining unknown input on residual. The fault detection problem of switched systems is equivalent to H_/H_∞ unknown input observers design. The average dwell time technique and multiple Lyapunov functions are utilized for deriving parameters of fault detectors, and further results are obtained by introducing the slack matrix method. A simulation example is given to illustrate the effectiveness of the proposed method.