The invention discloses a preparation method of purple sweet potato bean curd. The method comprises the following steps: a. cleaning: choosing beans with good quality, removing impurities, and cleaning the beans with water; b. soaking: soaking the beans with water according to the proportion that 1 kg of beans is added with 2.5 kg of water, for 4-5 hours in winter, and 2.5-3 hours in summer; c. grinding into pulp: mixing and grinding the soaked beans and the washed and peeled purple sweet potatoes with water according to the fact that the mass proportion of the beans to the purple sweet potatoes to the water is 1:0.5:6; d. filtering: separating out the prime pulp; e. boiling: pouring the prime pulp obtained in the step d into a pot to be boiled, then keeping the temperature for 5 minutes, and when boiling, adding fatty glyceride which is 1.0% of the weight of the prime pulp to obtain boiled purple sweet potato soybean milk; f. solidification: solidifying the obtained boiled purple sweet potato soybean milk; g. molding. The preparation method adds a beautiful color to the traditional food bean curd, while retaining various physiological activators in the purple sweet potato, so that the healthy bean curd is rich in nutrition; the preparation method is simple and feasible, and can be popularized in the market.