The invention provides a purple sweet potato red bean sauce and a preparation method thereof. The purple sweet potato red bean sauce is prepared from the following components in parts by weight: 100 parts of purple sweet potato paste, 5-10 parts of red bean paste, 16-24 parts of white granulated sugar, 0.09-0.12 part of citric acid, 0.11-0.14 part of calcium chloride, 0.06-0.12 part of pectin, 0.1-0.4 part of tea polyphenols, 5-8 parts of galactooligosaccharide and 10-15 parts of water. Compared with the prior art, the purple sweet potato red bean sauce disclosed by the invention has the advantages that purple sweet potatoes are used as main raw materials, and red beans are used as auxiliary materials, so that the produced composite sauce not only is rich in the content of nutrients of vitamins, minerals and the like, but also is rich in active components of soluble dietary fibers, anthocyanin and the like, has good effects of resisting oxidation, improving cytotrophy, strengthening constitutions, improving immunity of bodies and preventing diseases, and has good water preserving capability, and good product fluidity. The purple sweet potato red bean sauce is low in saccharose content, rich in fragrance, reasonable in nutrition, prominent in health-care functions and convenient to carry and eat.