The invention discloses a modified epoxy resin adhesive for a heat insulation rock wool board for an exterior wall of a building and a preparation method thereof. The modified epoxy resin adhesive comprises the components of epoxy resin, vinyl silane, thinner, polyorganohydrogensiloxane, curing agent and water in the mass ratio of 1: (0.02-0.03): (0.06-0.07): (0.03-0.04): (0.29-0.31): (0.15-0.17). The prepared modified epoxy resin adhesive has good water solubility, and the mixture ratio with the water can achieve 1: 3. The modified epoxy resin adhesive can enable a spray-bonded cotton product to realize uniform binding, the strength is higher than a commonly used rock wool adhesive, the stability is good, the environment-friendly property is excellent, a rock wool heat insulation layer has good waterproof and breathable functions, the strength of the heat insulation layer can be improved, and the service life of the heat insulation layer can be further prolonged.