Herein,　the　tape-casting　technique　was　used　to　fabricate　an　Er-doped　(K0.5Na0.5)NbO3　(KNN:0.02Er)　laminar　ceramic,　which　possesses　a　pseudo-cubic　phase　structure　with　regular　cube-shaped　grains.　Considering　the　substitution　behavior　of　the　Er　dopant　in　the　KNN　lattice,　X-ray　diffraction　analysis　and　density　functional　theory　simulations　both　verify　that　Er　can　replace　K　and　Na　sites.　And　Er-induced　enlargement　of　the　energy　band　gap　of　KNN:0.02Er　has　been　obtained　in　the　reflectance　spectra　and　simulations.　Furthermore,　the　ceramic　exhibits　dual-mode　down-conversion　and　up-conversion　photoluminescence　(PL).　As　a　lead-free　luminescent　ferroelectric,　the　PL　intensity　of　KNN:0.02Er　can　be　enhanced　by　polarization　induced　by　electric　poling.　And　the　dual-mode　PL　can　be　obviously　modulated　under　an　electric　field　in　an　in　situ,　reversible,　real-time　and　dynamical　way.　Our　results　offer　an　effective　theoretical　way　to　bridge　the　relationship　between　the　crystal　structure/chemical　bonding　environment　and　the　performance　of　the　KNN　system,　and　also　provide　an　opportunity　to　realize　electrically　controlled　tuning　of　the　PL　response　in　KNN-based　luminescent　ferroelectrics　and　the　corresponding　optoelectronic　devices.　©　2019　The　Royal　Society　of　Chemistry.