The invention discloses a preparation method for a cross-linked chitosan food preservative. The preparation method comprises the following steps: adding a chitosan raw material into acetic acid aqueous solution to obtain chitosan raw material solution; adding alpha-amylase for performing enzymolysis; cooling and filtering after the chitosan raw material solution is inactivated; adding water into the filtrate to dilute; adding Tween-80 into the diluted solution; mechanically stirring for emulsifying; adding iron sulfate into the emulsified diluted solution; adding sodium tripolyphosphate aqueous solution dropwise after the diluted solution and the Tween-80 are stirred and are reacted; stopping stirring after the sodium tripolyphosphate aqueous solution is added dropwise to prepare a cross-linked chitosan solution chelated with iron ions; adding table salt, stirring under the table salt is dissolved completely, and heating and drying under the water content is below 5 weight percent in vacuum, wherein the obtained powder is the cross-linked chitosan food preservative, and the dried cross-linked chitosan food preservative contains 2.5 mass percent of chitosan. The cross-linked chitosan food preservative has the benefits that the cross-linked chitosan serves as a food preservative and has high human security, and the sterilization effect reaches the existing national level of the food preservative which is approved for use.