The invention provides a Hylocereus undatus 'Foo-Lon' ferment jelly and a preparation method thereof, and relates to the food field. The preparation method includes the following steps: mixing 20-30 parts of Hylocereus undatus 'Foo-Lon' fruits, 5-8 parts of Siraitia grosvenorii fruits and 0.5-2 parts of honey to obtain mixed materials, and beating the mixed materials with water according to a ratio of material to liquid of 1:4-7, thereby obtaining mixed pulp; adding 0.05-0.2 part of compound probiotic into sterilized mixed pulp, and fermenting the mixture in a constant temperature environmentof 30-36 DEG C for 18-24 hours, thereby obtaining fermentation broth; and mixing the fermentation broth with jelly liquid at a volume ratio of 6-9:1, and then coagulating the mixture at 2-4 DEG C after sterilization. The Hylocereus undatus 'Foo-Lon' ferment jelly obtained by the preparation method has high nutritional value, smooth and refreshing mouthfeel and effects of moistening lung and clearing heat and improving gastrointestinal function.