The invention discloses a method for fermenting fish sauce from acaudina molpadioides semper. The method comprises steps as follows: (1), the acaudina molpadioides semper, shrimp heads and oyster meat are pulped, salt is added in the pulping process, and fermentation is performed for 3-5 days; (2), protease is added for hydrolysis for 5-8 days; (3), by-products of aquatic product processing are mixed with soy leaven, salt is added and stirred, and mixed fermentation is performed for 40-50 days; (4), aspergillus oryzae strains are inoculated, fermentation is performed for 40-50 days, and a fermentation broth is obtained through filtration; (5), yeast is added to the fermentation broth in the step (4), and then the fermentation broth is stirred uniformly and fermented for 40-50 days; (6), the fermentation temperature is decreased to 20-25 DEG C, and the fermentation is performed for 3-4 months; (7), filtering, sterilization and filling are performed, and a finished product is obtained. On the basis of combination of a traditional fish sauce fermentation technology, the method is improved, a low-salt state is kept in the whole fermentation process, and the hard problem of fishiness in fish sauce preparation is solved. With the adoption of the method, the fermentation time is short, and the prepared fish sauce has unique flavor, abundant nutrition and a certain healthcare function.