Perovskite nanocrystals are promising luminescent materials with synthetic feasibility and band gap tunability. Nonetheless, application of the perovskite nanocrystals to light-emitting devices has been challenging because of the intrinsic poor colloidal stability and environmental vulnerability issues. Here, we introduce a new protocol for highly air-stable perovskite nanocrystal layers with a tunable band gap via a simple nanocrystal pinning process. The nanocrystals were composed of CH3NH3PbBr3 (MAPbBr3) mixed with (vinylbenzylamine)2PbBr4 ((VBzA)2PbBr4), which contains a photopolymerizable structure-directing ligand. Along with the compostion of (VBzA)2PbBr4, the band gap of the perovskite layer continuously increased with the reduction of the nanocrystal size and also lattice distortion. The nanocrystal film readily polymerized upon exposure to visible light was highly stable under humid air more than 15 days. Its application to bluish-green light-emitting diodes is demonstrated.