The invention provides a preparation method of N-acyl acidic amino acid or a salt thereof, comprising subjecting a fatty acyl chloride and an amino acid to an amidation reaction under an alkaline condition. The preparation method is characterized in that in the amidation reaction, water is used as a solvent, an acidic amino acid or a salt thereof is used as a main raw material and a small amount of a neutral amino acid or a salt thereof is used as an auxiliary raw material, and the method comprises the following steps: under a stirring condition, firstly adding the fatty acyl chloride dropwise into an aqueous solution of the acidic amino acid or the salt thereof； adding an alkali to adjust the pH value of the reaction solution； after a certain amount of fatty acyl chloride having been added dropwise, adding an aqueous solution of the neutral amino acid or the salt thereof, and continuing to add the fatty acyl chloride dropwise until the addition is finished and stirring to maintain the reaction. The preparation method uses the reaction of the mixed amino acids and the fatty acyl chloride under a water-phase system, so that the conversion rates of the amino acids and the acyl chloride can be remarkably increased and the amount of residual amino acids is greatly reduced. The product can be directly used as a surfactant after simple post-treatment, and thus the cost is greatly reduced.
