The invention relates to an in-situ synthesized metal coating silver brazing rod and a preparation method thereof. The in-situ synthesized metal coating silver brazing rod is composed of a strip-shaped silver brazing filler metal core and a coating coated on the outer part of the silver brazing filler metal core, wherein the silver brazing filler metal core consists of the following components inpercentage by weight: 10-50% of Ag, 15-45% of Cu, 17-40% of Zn, 1-3% of Sn, 0.1-0.5% of P, 0.5-2% of Ni, 0.1-0.3% of Si and 0.1-0.3% of Li; and the coating consists of the following components in percentage by weight: 5-10% of elemental boron micro powder, 5-10% of sodium borohydride, 15-30% of potassium fluoborate, 25-40% of boric anhydride, 10-30% of sodium fluoride, 2-4% of potassium bifluoride, 1-5% of copper sulfate, 0.1-2% of polystyrene and 0.2-2% of dibutyl phthalate. The brazing filler metal core of the coating sliver brazing rod is good in wettability, is good in liquidity, has self-brazing effect, and enables zinc to be not liable to volatilize; and moreover, the coating has the advantages of being high in activity, weak in moisture absorption, less in carbon residues, good in plasticity and toughness, and the like, and is especially suitable for assemblies such as brazing stainless steel and manganese brass.