Nonlinear　multi-phonon　(2-7)　absorption　in　the　Na+/Bi3+-alloyed　Cs2AgInCl6　lead-free　double　perovskites　with　similar　to　100%　photoluminescence　quantum　yield　and　superior　stability　is　observed　for　the　first　time,　which　can　be　pumped　by　a　femtosecond　laser　in　a　wide　spectral　range　(800-2600　nm).　First-principles　calculations　verify　that　the　parity-forbidden　transition　from　the　valence　band　maximum　and　conduction　band　minimum　(at　the　Gamma　point)　is　not　broken　by　Na+/Bi3+　doping,　and　strong　optical　band-to-band　absorption　occurs　at　the　L&X　points.　Time-resolved　emission　spectra　evidence　that　single-photon　and　multi-photon　pumping　leads　to　the　same　self-trapped　exciton　transition　and　high-order　nonlinear　absorption　will　not　induce　a　remarkable　thermal　effect.　Finally,　we　demonstrate　that　the　Cs2Na0.4Ag0.6In0.99Bi0.01Cl6　DP　shows　great　potential　for　next-generation　wavelength-selective　and　highly　sensitive　multiphoton　imaging　applications.