The invention discloses a single-component low-viscosity medium-temperature fast curing epoxy resin adhesive and a preparation method thereof, the single-component low-viscosity medium-temperature fast curing epoxy resin adhesive comprises the following components by weight: 35-45 parts of epoxy resin, 30-40 parts of an anhydride curing agent, 15-30 parts of a filler, 0.1-1 part of a quaternary ammonium salt accelerator, and 1-5 parts of an auxiliary agent, and the auxiliary agent is one or more of a coupling agent, an antifoaming agent and a leveling agent. The preparation method comprises the following steps: mixing the epoxy resin and the auxiliary agent, uniformly stirring at 500-1000rpm, then adding the filler, stirring at 1500-2500rpm for 10-30 minutes, then adding the anhydride curing agent and the quaternary ammonium salt accelerant, stirring at 800-1500rpm for 30-50 minutes, filtering, and carrying out vacuum defoaming at 30-50 DEG C for 0.5-1.5 hours to obtain the single-component low-viscosity medium-temperature fast curing epoxy resin adhesive. The epoxy resin adhesive disclosed by the invention is a single-component system, has the advantages of low viscosity, good leveling property, medium-temperature rapid curing and the like, and can be applied to the fields of electronic packaging, positioning and the like.