The invention discloses a preparation method of jelly for preventing tooth decay. The preparation method comprises crushing xylitol, flaxseed, Ficus pumila Linn., citric acid, carrageenan, agar, gelatin and konjac flour to obtain primary mixed powder, dissolving the powder in 1000 parts of water, heating the mixture, adding food-grade calcium phosphate into the solution, carrying out uniform mixing, carrying out cooling to obtain a jelly product, freezing and refrigerating the jelly product, heating to melt the intermediate mixture, adding seasoning powder into the mixture, carrying out cooling for recondensing to obtain transparent gel, freezing and refrigerating the transparent gel, heating to melt the transparent gel, adding dietary fibers into the transparent gel melt, adding lactic acid bacteria into the melt at 38-43 DEG C, carrying out uniform mixing to obtain mixed gel and carrying out sterilization to obtain jelly for preventing tooth decay, wherein a mass ratio of lactic acidbacteria to dietary fibers is 1-2: 10. The jelly contains a caries-preventing component and does not produce dental caries when a large amount of the jelly is eaten.