The invention provides functional Ziyu Chinese yam and fruit composite juice and a preparation method thereof, belonging to the technical field of fruit juice processing. The composite juice is prepared from the following raw materials in parts by weight: 20-35 parts of Ziyu Chinese yam, 8-15 parts of red pitaya pulp, 10-18 parts of mulberry fruits, 8-15 parts of red beans, 12-20 parts of color fixative, 2-5 parts of stabilizer, 15-50 parts of sweetening agent, 2-8 parts of acidity agent, 0.2-0.3 part of alpha-amylase, 0.2-0.4 part of glucoamylase, 0.15-0.25 part of cellulase, 0.15-0.25 part of protease, 0.02-0.03 part of pectase and 360-650 parts of water. The fruit juice has efficacy of nourishing yin, supplementing blood, tonifying spleen, nourishing stomach, maintaining beauty and keeping young. With the adoption of the preparation method, nutrition and fragrance of each component can be well kept. A purple color can be kept due to the addition of the suitable color fixative. In addition, the produced beverage is uniform, fine and smooth in texture, and is sour, sweet and delicious in taste without precipitation.