An under water decorative light device is composed of solid luminous unit array fixed on substate, box for containing the array and substrate, sealing filler solidified in cavity of box, cable connected electrically to luminous unit array, and fly's eye type of lens array as light-guide cover plate of box. its advantages are low energy consumption and generated heat, high luminosity and projection effect, high safety, long service life, high response, and can be used along with music fountain.