The present invention relates to an oat-sesame milk and a preparation method thereof, wherein oat, milk powder and black sesame are adopted as main raw materials and are subjected to blending, mixing, homogenizing, filling and UHT sterilization to obtain the oat-sesame milk. The raw materials of the oat-sesame milk comprise, by weight, 50-60 parts of oat, 5-7 parts of black sesame, 10-12 parts of milk powder, 6-7 parts of white granulated sugar, 400-550 parts of water, 0.05-0.2 part of carboxymethyl cellulose, 0.03-0.1 part of propyleneglycolalginate, 1-3 parts of xanthan gum, 2-5 parts of sucrose fatty acid ester, 0.03-0.1 part of molecular distilled monoglyceride, and 0.01-0.05 part of a compound phosphate. According to the present invention, the oat-sesame milk is a composite beverage with characteristics of unique flavor, rich nutrition and good taste, the oat nutrition value in the components is high, the oat is the good processing raw material, the black sesame has rich aroma and is the good medicine with effects of disease elimination and health enhancing, and compared with the simple milk, the oat-sesame milk has the following advantages that: the protein ratio is close to the breast milk, the fat content is reduced, and the cost is saved.