The　structural　stability,　electronic　and　optical　properties　of　Rb-incorporated　methylammonium　lead　halide　perovskite　(MAPbI(3))　were　investigated　by　first-principles　calculations.　A　high　fraction　of　Rb+　incorporation　can　degrade　the　phase　stability　of　MAPbI(3).　Moreover,　the　band　gap　of　Rb-doped　MAPbI(3)　exhibits　slightly　blueshifted.　The　optical　absorption　of　Rb-doped　MAPbI(3)　is　almost　the　same　with　that　of　the　undoped　MAPbI(3),　which　can　be　attributed　to　the　close　band　gaps　of　them.　Our　work　can　agree　well　with　existing　experimental　results.　Our　calculated　results　can　provide　a　basic　insight　into　the　mixed-cation　halide　perovskites　for　optoelectronic　applications.