The invention provides a preparation method of mulberry leaf dark tea. The method comprises the following steps of: picking fresh annual mulberry leaves, soaking the leaves in a soaking barrel for 2-3days, fishing out the leaves, juicing the leaves in a squeezer to obtain mulberry leaf juice, selecting tea leaves according to a conventional process, performing fixation, adding the mulberry leaf juice into the tea leaves during rolling, putting the rolled tea leaves into a fermentation barrel, performing steam heating on the fermentation barrel, pouring out the heated tea leaves, cooling the leaves to room temperature, and performing pile fermentation; and drying and molding the fermented tea leaves. The method has the advantages that the mulberry leaves are not in contact with and mixed with the tea leaves, the quality of the tea leaves is not affected during storage, meanwhile, the nutrients of the mulberry leaves can be perfectly fused into the tea leaves, and the fused dark tea isgood in taste and rich in nutrition.