A series of Norrish type I photoinitiators (NASs), which are naphthalimide aryl sulfide derivatives, are prepared. The potential mechanism involved in the photolysis of NASs under UV LED at 405 nm is investigated by steady-state photolysis, nuclear magnetic resonance, electron spin resonance, fluorescence spectroscopy, cyclic voltammetry, and laser flash photolysis and by calculating the bond dissociation energies of the C-S bonds of NASs. The as-prepared photoinitiators NASS and NAS6 can efficiently initiate free radical photopolymerization of (methyl)-acrylate monomers under UV LED exposure at 405 nm. NASs/iodonium salt systems can initiate the cationic photopolymerization of epoxide. Interestingly, NASs exhibit an excellent stability to sunlight. Thus, the use of NASs makes the preparation, storage, and application of photocurable formulations convenient.