The　optimal　control　problem　of　the　switched　system,　which　subsystem　is　discrete　linear　system,　is　investigated.　In　the　conditions　of　the　given　switched　number　and　the　quadratic　performance　function,　the　multi-periods　decision-making　problem　can　be　resolved　by　being　transformed　into　single　decision-making　problem.　Genetic　algorithm　is　used　to　search　the　switched　instant　to　minimize　the　performance　function　so　that　the　optimal　control　problem　of　the　switched　discrete　system　can　be　obtain　a　analytic　solution　in　global.　A　numerical　example　illustrates　the　effectiveness　of　the　design　approach.　The　simulation　shows　that　the　approach　can　resolve　the　optimal　control　problem　of　a　kind　of　switched　discrete.