The invention discloses an enzyme and a preparation method thereof. The preparation method comprises the following steps: a, preparing a raw stock liquid, namely weighing multiple fruits, multiple vegetables and multiple edible mushroom raw materials, and making the raw stock liquid; b, making a fermentation stock, namely respectively inoculating four probiotics, namely lactobacillus acidophilus,bacillus, brewer's yeast and bifidobacterium into a culture medium so as to obtain the fermentation stock, wherein the culture medium comprises 75-80 parts of water, 2-5 parts of black sugar and 10-15parts of sweet potatoes; c, making diluted mixed slurry, namely adding pure water which is 4-8 times of the weight of the raw stock liquid obtained in the step a as a standard, licorice roots which are 0.01-0.1 time of the weight of the raw stock liquid, and black sugar which is 0.1-0.2 time of the weight of the raw stock liquid, boiling, keeping boiling for 4-6 minutes, stopping heating, and cooling so as to obtain the diluted mixed slurry; d, making a fermentation broth, namely inoculating the fermentation stock of the step b into a fermentation tank of the diluted mixed slurry of the stepc in a ratio of 0.01-0.05 times of the weight of the diluted mixed slurry, and fermenting for 80-100 days in a sealed environment. By adopting the technical scheme of the invention, the content of amino acid in the enzyme can be greatly increased.