The invention relates to an underwater epoxy concrete, mortar additive and a preparation method thereof. The additive comprises component A and component B which comprise the ingredients by weight percent: 40-45% of epoxy resin E-51, 16.5-21% of epoxy resin E-44, 9-13% of reactive diluent, 6.0-7.5% of toughening agent, 1-2% of coupling agent, 20-24% of curing agent T-31; wherein the curing agent T-31 belongs to the component B, and the rest ingredients are component A. The method comprises: the epoxy resin E-51 and the epoxy resin E-44 are heated to be about 40 DEG C and then evenly stirred; Kocsis toughening agent QS-BE is added; after that, the diluent and the coupling agent are added, and the mixture is evenly dispersed by a high-speed disperser, so that the component A can be obtained; the component A and the curing agent T-31 are mixed together and evenly stirred, so that the underwater epoxy concrete, mortar additive can be obtained. The additive is simple in preparation technique, and the prepared mixture has no phenomena of segregation and dispersion, has the functions of self-leveling and self-compaction and needs no concrete vibrating when being cast in the water; after being cured, the additive has good splicing performance with the original concrete; the prepared mixture has strong acid and alkali corrosion resistance after being cured in the water; therefore, the invention can be applied to underwater casting of concrete structures such as bridge piers, dams and bottom plates of reservoir power stations and the like as well as underwater thin-layer repair.