The invention relates to a benzimidazole diamine curing type epoxy adhesive and a preparation method thereof. The adhesive is prepared from the following raw materials: a component A, namely a benzimidazole diamine curing agent and a component B, wherein the component B is prepared from terminal carboxyl group butadiene-nitrile rubber, epoxy resin and a reactive diluent. The preparation method comprises the following steps of: (1) adding the terminal carboxyl group butadiene-nitrile rubber and the epoxy resin into a reaction kettle, reacting at the temperature of 90 DEG C with stirring, cooling to the temperature of 50 DEG C, adding the reactive diluent, and stirring at the temperature of 80 DEG C to obtain homogeneous and transparent viscous liquid, namely the component B; and (2) mixingthe components A and B, and stirring uniformly to obtain the adhesive. The tensile shear strength of the adhesive is 33.5MPa at the room temperature, and the adhesive has wide application prospect infields of electronics and microelectronics, rigid copper-clad laminates, motors, aerospace and the like; and the preparation process is simple, is low in cost and is convenient to operate, the resources of reaction raw materials are wide, and the adhesive can be conveniently industrially produced.