The invention discloses a vacuum circuit breaker, which comprises a first outer shell, a second outer shell, a fixed contact, a movable contact, an air faucet, a piston, a seal cover and a buffer device, wherein the fixed contact and the movable contact are arranged in the first outer shell; the inside of the first outer shell is vacuumized; the first outer shell is located in the second outer shell; the first outer shell and the second outer shell have a common top surface; a hollow cavity between the first outer shell and the second outer shell is vacuumized; the buffer device comprises a first round sheet and a second round sheet; a plane in which the first round sheet is located is parallel to the plane in which the second round sheet is located; and the first round sheet is connected with the second round sheet through a plurality of springs. By the buffer device and the structure of the first outer shell and the second outer shell, the air tightness can be greatly improved.