The invention discloses a red bean and chocolate liquid milk product and a preparation method thereof. The red bean and chocolate liquid milk product includes the following materials of: 28-98% of raw milk, 0.001-6% of sweet substance, 0.1-0.35% of thickening agent, 0.01-0.21% of emulgator, 0.5-5% of red bean puree and / or red bean powder, 0.3-2% of cocoa powder, 0.1-1% of cocoa liquor, 0.01-1% of dietary fiber, 0.01-0.17% of composite salt, 0.02-0.1% of edible essence and the balance of water. The preparation method includes the steps of: 1, adding the cocoa powder and the cocoa liquor, and mixing to obtain cocoa solution; 2, preheating the raw milk, adding the sweet substance, the thickening agent, modified starch and the emulgator, and then mixing to obtain solution A; 3, mixing the cocoa solution with the solution A, homogenizing, sterilizing and cooling, and thereby obtaining the product. The red bean and chocolate liquid milk product is particular in flavor, rich in nutrition, and good in stability.