The present invention relates to a method for the preparation of chitosan of a degree of acetylation DAr, from chitosan β having a degree of acetylation DAi lower than DAr, comprising at least one step of purification and at least one step of acetylation of said chitosan β, said purification step comprising at least one step of solubilization of said chitosan β in an acidic aqueous medium and at least one step of precipitation of said thus solubilized chitosan, wherein said acetylation step is carried out in a solution on said solubilized chitosan before said precipitation step.