The present invention provides a dietary fiber composition and a preparation method thereof. The dietary fiber composition comprises the following components: 5.5 to 8.5 parts by weight of resistant dextrin and 0.7 to 1.5 parts by weight of Xylooligosaccharides; the preparation method of the dietary fiber composition is as follows: 1, mixing 5.5 to 8.5 parts by weight of the resistant dextrin and 0.7 to 1.5 parts by weight of the xylooligosaccharides to completely dissolve in water to obtain a liquid A; (2) using spray granulation method to prepare the liquid A into a finished product. The beneficial effects are that the dietary fiber composition has obvious effects in promoting the proliferation of Bifidobacterium, reducing the number of clostridium septicum, lowering blood fat and fasting blood sugar, and improving defecation effect, and has a certain effect in reducing body weight.