The invention provides a preparation method of carbon nanodots with effective wide absorption. The method comprises the following steps: dissolving urea and polycarboxyl compounds in a high-boiling-point organic solvent, and heating under closed conditions to react to obtain the carbon nanodots with effective wide absorption. The carbon nanodots are prepared from the urea and polycarboxyl compounds by using the high-boiling-point solvent; and thus, the preparation method is simple, and the preparation raw materials are cheap. Therefore, the obtained carbon nanodots have effective wide absorption peak in the whole visible light region. In addition, the prepared carbon nanodots have favorable solubility in water and organic solvents, and thus, are convenient for solution processing. Besides, the carbon nanodots have favorable crystallinity, thereby being beneficial to developing photoelectric properties of the carbon nanodots.