In this work, the in situ reduction of Ag+by carbon quantum dots (CQDs) to the formation of CQDs-silvernanocomposite (CQDs-silver NPs) was exploited to develop a facile and ultrasensitive colorimetric sensorplatform. Due to the high affinity between Ag and melamine, melamine could significantly induce thewell-dispersed CQDs-silver NPs aggregation. Consequently, the solution color of CQDs-silver NPs changedfrom bright yellow to red. Based on this finding, we proposed a new colorimetric method to qualitativelyand quantitatively detect melamine by the naked eyes and UV–vis spectroscopy, respectively. Under theoptimum conditions, the linearity of melamine with the solution absorbance (_R) was plotted in the con-centration range of 1 × 10⁻¹⁰–1 × 10⁻⁸g/mL (y = 0.55 + 0.049x, R = 0.9596) and 2.5 × 10⁻⁸–5 × 10⁻⁷g/mL(y = 3.43 + 0.42x, R = 0.9961). The developed method showed excellent detection sensitivity for melaminewith the detection limit as low as 7.9 × 10⁻¹²g/mL (eq. 65.3 pmol L⁻¹). Finally, the developed method wasemployed to detect the amount of melamine in cow milk and milk powderand a recovery of 87.6–116%,and RSD of 1.6–3.9% was achieved.