The invention discloses a preparation method of monocrystal rock sugar containing lycium barbarum polysaccharide. The preparation method comprises the following steps: S1, preparation of sugar liquor:dissolving white granulated sugar and water, filtering by using a screen, and removing insoluble substances in the sugar liquor to obtain syrup to be used; S2, preparation of fructus lycii liquor: boiling fructus lycii and water in boiling water for 1.5 h, and then removing insoluble substances to obtain the fructus lycii liquor to be used; S3, crystallizing and feeding seed crystal in three stages, correspondingly adjusting the temperature and vacuum degree in a vacuum crystallization tank, and carrying out crystallization and crystal growth for 38-40 h; and S4, centrifuging, separating liquor, meanwhile rinsing with purified water, and washing away a sweet film attached to crystal to obtain crystal to be dried; and drying and packaging to obtain the final product. According to the invention, three-stage crystallization is adopted in the crystallization process, and after variables are reasonably controlled, the crystallization process is shorter than conventional crystallization by2-3 hours, and meanwhile, the monocrystal rock sugar has obvious advantages in the aspect of reduction of sugar.