The invention discloses a composite bamboo shoot fermenting agent and a preparation method of fermented bamboo shoots. The composite bamboo shoot fermenting agent contains monascus vinasse and liqueur koji. The preparation method of the fermented bamboo shoots comprises the following steps of: firstly, blanching bamboo shoots; and then, fermenting the bamboo shoots under the action of the composite bamboo shoot fermenting agent. Under the action of the composite bamboo shoot fermenting agent, aromatic substances, pigments, and the like in the vinasse are infiltrated in the bamboo shoot tissues. The prepared vinasse-processed bamboo shoots have the characteristics of unique fragrance and flavor, mellow and pleasant taste, elegant flavor, attractive color and rich nutrition. The preparation method can effectively prevent fresh bamboo shoots from aging, lessen the loss of nutrient contents, preserve the nutrient contents of the bamboo shoots and ensure the quality and the grade of the bamboo shoots. The fermented bamboo shoots prepared by using the method disclosed by the invention have appropriate proportions of liquor, sugar and acid, and have the advantages of low alcohol and sugar contents, stable resistance to acid and alkali, strong heat resistance and light resistance, stable resistance to oxidoreduction and favorable practical values and application prospects.