The invention belongs to the technical field of bait preparation, and particularly relates to slow-release composite bait and a preparation method thereof. The slow-release composite bait is prepared from the following components in parts: 40-45 parts of high gluten flour, 30-36 parts of spirulina powder, 5-10 parts of distillers' grains, 22-26 parts of fructus cannabis, 10-15 parts of yeast powder, 35-40 parts of water, 15-20 parts of termites, 8-10 parts of yellow mealworm, 6-10 parts of earthworm, 15-18 parts of fish meal, 12-16 parts of caterpillar, 6-8 parts of radix angelicae, 7-12 parts of astragalus membranaceus, 9-15 parts of amomum tsao-ko, 5-9 parts of angelica sinensis, 3-7 parts of liquorice, 18-22 parts of n-butyl alcohol and 20-25 parts of a slow-release carrier. The fish bait is balanced in formula nutrition, safe, pollution-free and low in manufacturing cost, so that the production cost for fishing or capturing fishes is greatly reduced; and meanwhile, all the fish bait can be decomposed by the natural environment, the water resource and the environment are obviously protected and improved, and environmental pollution and damage to human beings, fishes and water areas where the human beings, the fishes and the water areas where the fishes live are avoided.