An LED illuminating device comprises a housing with a base plate, a prism sheet and a circuit board. The prism sheet is mounted in the housing and comprises a number of V-shaped micro-grooves formed thereon along three directions intersecting with each other. The circuit board is fixed on the base plate, wherein a plurality of LED units are arranged on the circuit board, each two adjacent LED units are spaced a preset distance apart from each other and emit light of different colors. Six virtual mixed light sources are formed by reflection of the prism sheet to the circuit board when the light emitted by an LED unit mixes with light emitted from adjacent LED units on the circuit board. Each of the virtual mixed light sources has a color different from the colors of the light emitted from the LED units.