Abstract: In this study, kelp juice, apple juice and sugar were used as the main raw materials, and Lactobacillus plantarum was applied as the fermentation strain, to develop a fermented kelp-apple compound juice beverage with excellent flavor and natural nutrition. Based on sensory scores, the process optimization of the compound fruit juice beverage was carried out through single factor experiments and orthogonal experiment design. The best formula and fermentation condition of the beverage were as follows: 1:2 volume ratio of kelp juice to apple juice, 3% sucrose addition, 8% Lactobacillus plantarum inoculation, fermentation time 24 hours and temperature 32 ℃. The result of antioxidant activity assay showed that the inhibition rate of DPPH free radical, ABTS⁺ free radical and total antioxidant capacity of fermented kelp-apple juice beverage were 73.20%, 36.05% and 0.774, respectively. On the whole, both kelp juice and apple juice showed certain antioxidant capacity. Fermentation improved the antioxidant capacity of kelp juice, but reduced the antioxidant capacity of apple juice. However, the fermentation of sole kelp juice had poor taste and flavor. The addition of apple juice and lactic acid produced by fermentation masked the fishy smell of kelp to a certain extent. On the other hand, the addition of kelp juice prevented decline of antioxidant capacity of sole apple juice fermentation. Fermentation of two juices could significantly improve the DPPH free radical scavenging capacity of beverage. This study solved the problem of fishy smell of kelp beverage, explored the antioxidant activity of kelp-apple juice beverage fermentation system, and would provide theoretical basis and practical guidance for the development of kelp-fruit juice beverage.