A composite heat-dissipating device is provided for significantly increasing the number and size of blades so as to enhance the heat-dissipating performance. The composite heat-dissipating device is constructed by a plurality of impellers, each of which includes a plurality of blades. When the plurality of impellers are assembled together, the plurality of blades are arranged around the hub of the composite heat-dissipating device and there is an overlapped region formed between every two adjacent blades.