A sunlight communication system is proposed that uses Sr₂Si₅N₈:Eu² phosphors to concentrate sunlight signals in strong background light noise; thus, a wide spectrum sunlight communication system is converted into a narrow spectrum one. A communication method is proposed to enable compression to the dark line H-α (656.28 nm) spectrum. A 50% solar energy conversion efficiency is achieved with a 0.3 μs code delay, a 0.2 μs code rise time (20%–80%), and a 96% optical transmittance. Experimental results show that phosphors enhance the sunlight intensity 1.5 times with the same distance. This method has immense potential in future long-distance sunlight communication.