The invention provides a heat dissipating device for an LED illuminating system. The heat dissipating device comprises a sealing base plate, the sealing base plate is sequentially provided with phasechange cavities and a cooling cavity, and a phase change material is loaded in the phase change cavities. The cooling cavity is provided with a cooling water inlet and a cooling water outlet, a constant temperature box is fixedly arranged on the cooling cavity, and a constant temperature box water inlet and a constant temperature box water outlet are formed in the constant temperature box. The constant temperature box water outlet and the cooling water inlet communicate through a first miniature circulating pump, the constant temperature box water inlet and the cooling water outlet communicatethrough a second miniature circulating pump, and a holding table is further arranged on the cooling cavity. The first miniature circulating pump and the second miniature circulating pump are locatedon the holding table. The heat dissipating device has the advantages that the structure is simple, the heat dissipating effect is good, and the service life is long.