The invention discloses carboxymethyl chitosan nano medicament-carrying microspheres and a preparation method thereof. The preparation method comprises the following steps of: preparing aqueous solution of carboxymethyl chitosan, dissolving a medicament into the carboxymethyl chitosan solution, stirring the solution uniformly, adding calcium chloride solution, and performing one-time ion cross-linking pre-coating; adding surfactant into an oily component, stirring the surfactant and the oily component to obtain an oil phase mixture, adding the pre-coated mixed solution into the oil phase mixture, quickly stirring the solution to form a water-in-oil mixture, adding glutaraldehyde, performing secondary curing cross-linking reaction to obtain medicament-coated carboxymethyl chitosan microspheres, performing centrifugal separation on the two-phase mixture, cleaning the lower layer by using petroleum ether or ethanol solvent, washing with distilled water, drying, and thus obtaining the carboxymethyl chitosan nano medicament-carrying microspheres. Because the carboxymethyl chitosan with biocompatibility, safety, non-toxicity and capability of being safely metabolized in vivo is used as a coating material, the carboxymethyl chitosan nano medicament-carrying microspheres have the characteristics of small medicament-carrying granule, high medicament-carrying amount and coating rate, good sustained-release performance and capability of fully exerting the effect of the medicament.