High　performance　blue　light-emitting　diodes　are　indispensable　for　solid-state　lighting　and　full-color　displays.　Recently,　tremendous　efforts　have　been　dedicated　to　boost　the　performance　for　blue　perovskite　light-emitting　diodes　(PeLEDs).　In　this　work,　we　construct　an　ingenious　2D/3D　heterostructure　perovskite　by　long-chain　organic　halide　salt　post-treatment　in　blue　perovskites　for　the　first　time.　Organic　halide　salt　post-treatment　can　form　a　2D　capping　layer　on　the　original　3D　perovskite　surface,　thus　to　passivate　defects　and　improve　exciton　radiative　recombination　rate.　As　a　result,　2D/3D　PeLEDs　with　tunable　blue　emissions　(465,　475,　and　486　nm),　ultra-low　turn-on　voltages　(3.0,　2.7,　and　2.7　V),　high　brightness　(928,　6727,　and　9242　cd　m(-2)),　and　excellent　EQEs　(2.11%,　6.29%,　and　8.57%)　are　demonstrated.　Moreover,　due　to　the　high　phase　homogeneity,　the　optimized　PeLEDs　exhibit　ultrapure　blue　emissions　(FWHM　＜　15　nm),　in　which　the　PeLED　(465　nm)　achieve　&　AP;98%　coverage　of　the　Rec.　2020　standard.