The invention provides a preparation method for carbon dots. The preparation method comprises the steps: mixing citric acid and urea in a solvent, and then, carrying out heating, so as to obtain a reaction solution; and carrying out centrifugal separation on the reaction solution, so as to obtain precipitates, i.e., the carbon dots, wherein the solvent is one or a mixture of two of water, glycerine and dimethylformamide. According to method, visible-light-all-waveband-luminescent carbon dots are prepared through adopting different solvents, and thus, the carbon-dot composite material provided by the invention can emit light in all wave band of visible light, particularly, white-light carbon-dot composite materials of different color coordinates and color temperatures can be obtained by adopting the carbon dots of different colors. Furthermore, the carbon-dot composite material provided by the invention employs silicon dioxide as a dispersion matrix, so that gathered induced fluorescence quenching can be inhibited, and the composite material has relatively high quantum efficiency. The invention further provides the preparation method for the carbon dots, the carbon-dot composite material, a preparation method therefor and a luminescent LED (Light Emitting Diode).