The invention relates to an adjuvant for enhancing a fish vaccine immunization effect and an application thereof. The adjuvant for enhancing the fish vaccine immunization effect is characterized in that an immune potentiator is extracted from Astragalus mongholicus as a leguminous plant or effective components of the Astragalus mongholicus, comprising Astragaloside and Astragalus polysacharin, and natural products or manually modified products or manually synthetic products comprising the Astragaloside and the Astragalus polysacharin can be adopted to serve as the effective components. The adjuvant is capable of increasing the specific immune protection rate after vaccine component immunization is finished. When the adjuvant is applied, a vaccine can comprise the following components: any one or more than one of expression products of inactivated pathogens, bacterial ghost components, hypotoxic pathogens, attenuated pathogens, protective antigens, antigen subunits, antigen determinants or antigen gene expression vectors of bacteria, viruses and parasites. When in use, the adjuvant can be mixed with the components of the vaccine for application, also cannot be mixed with the components of the vaccine for application and also can be applied with the vaccine at different time.