The invention discloses electric vacuum brazing filler metal and a preparation method thereof. The brazing filler metal comprises the chemical components of, by mass, 30-55% of silver, 1-6% of gallium, 0.5-3% of indium and the balance copper. According to the brazing filler metal, a brazing filler metal thin strip and a brazing filler metal wire are prepared by adopting vacuum alloying smelting, sheet billet and rod continuous casting through protective gas and a traditional process. According to the AgCuGaIn alloy brazing filler metal prepared through the method, the melting temperature rangeis 760-810 DEG C, the product spatter property reaches the first grade, the cleanability reaches the first-second grade, the property consistency is good, the production efficiency and the finished product rate are high, the product can replace a AgCu28 alloy and a AgCuNi28-1 alloy to be applied to brazing of a vacuum circuit breaker, and the cost of the product is lower than the AgCu28 alloy anda AgCuNi28 alloy by 10-20% or above.