By　simply　doping　with　a　certain　amount　of　Sm3+,　opaque　lead-free　(K0.5Na0.5)NbO3　piezoelectric　ceramics　were　fabricated　into　transparent/translucent　ceramics.　The　photochromic　(PC)　behavior　and　associated　reversible　transmittance/luminescence　modulation　were　realized　via　physical　means.　Upon　illumination　under　a　xenon　lamp,　the　maximal　decreased　values　of　optical　transmittance　and　luminescence　intensity　are　36.1%　and　49%,　respectively.　Meanwhile,　the　colors　of　all　the　ceramics　turn　darker　after　illumination,　and　subsequently　return　to　their　initial　states　by　thermal　stimulus　(200　°C　for　5　minutes),　exhibiting　a　typical　PC　phenomenon.　Combining　experimental　and　calculation　results,　it　can　be　found　that　the　PC　mechanism　is　closely　related　to　the　vacancy　defects　induced　by　both　higherature　sintering　and　Sm3+-doping.　Additionally,　the　Sm3+　effects　on　phase　structures,　microstructures,　optical　transmittance　and　photoluminescence　properties　of　the　ceramics　were　systematically　studied.　The　substitution　behavior　of　Sm3+　and　vacancy　defect　concentrations　on　the　energy　band　gap　of　the　material　were　unraveled　by　the　first　principles　calculations　to　further　clarify　the　corresponding　mechanism.　The　Sm3+-doped　(K0.5Na0.5)NbO3　transparent　ceramics　have　potential　for　application　in　modulation　and　conversion　of　photoenergy,　such　as　in　optical　memory　and　photo-switching　devices.　And　the　reversible　tuning　of　transmittance　in　our　material　is　especially　attractive　for　exploring　novel　transparent　PC　materials.　©　2019　The　Royal　Society　of　Chemistry.