A method for preparing an aqueous suspension of nanocapsules comprising an oily core surrounded by a polymeric shell, comprises mixing first and second phases, wherein the first oily phase comprises a hydrophobic polymer, an oil or a mixture of oils, at least one active ingredient, and a surfactant TA1. The oily phase is brought to a temperature T1 higher than the melting point of the hydrophobic polymer, the hydrophobic polymer being miscible, at this temperature T1, with the mixture of the surfactant TA1 and the oil or mixture of oils, and the active ingredient being miscible, soluble or solubilized in the mixture of the surfactant TA1 and the oil or mixture of oils. The second polar phase comprises a hydrophilic polymer in the form of a hydrogel in an aqueous solution containing a surfactant TA2, to form the nanocapsules in an aqueous suspension.