The invention relates to the field of food processing, and particularly discloses fermented soy sauce and a preparation method thereof. The preparation method comprises the following steps: a, mixing the soaked soybeans with the wheat kernels, and then curing to obtain a mixture; b, at the temperature of 35-40 DEG C, mixing the mixture, koji and flour for koji making to obtain soy sauce koji, then uniformly mixing the soy sauce koji with a salt solution to obtain fermented mash, controlling the temperature of the fermented mash to be 45-50 DEG C, performing early-stage fermentation, then adding the salt solution, continuing later-stage fermentation, pumping out soy in the fully fermented mash, and performing solarization under light, thereby obtaining the fermented soy sauce. The fermented soy sauce prepared by the preparation method disclosed by the invention is relatively high in reducing sugar content, relatively high in amino acid nitrogen content, relatively high in palatable taste degree, bright in color and luster, rich and mellow in sauce flavor, natural in fresh, sweet and salty mixture, free of additives such as a preservative, a flavor enhancer, a sweetening agent and a coloring agent, and is healthy and safe.