Traditionally, it is time-consuming to synthesise large-sized quantum dots (QDs) in the aqueous way. Herein, a fast and facile method for aqueous QD synthesis was developed. By simply diluting a high-concentration precursor into low concentration with deionised water, QDs can nucleate and grow rapidly after heating. The influences of the dilution ratio, original precursor concentration and different ligands on the synthesis speed have been studied. Besides, the mechanism for the fast-growth rate has also been elucidated.