The utility model discloses an explosion chamber for a miniature circuit breaker, comprising an arc isolating cover enclosed by a left side board, a right side board and a back panel and multiple arc extinguishing grid plates vertically arranged and inserted into the arc isolating cover. Each arc extinguishing grid plate is provided with a gap having the same depth from outside to inside, the gap is composed of a rectangular opening segment in the outer portion, a splay segment in the middle portion and a U shaped segment in the inner portion, the central line of the outer portion segment of the gap is overlapped with the central line of the arc extinguishing grid plate perpendicular to the back panel, and the central line of the inner portion segment of the gap is parallelly located in the left side or right side of the central line of the arc extinguishing grid plate perpendicular to the back panel, such that the inner portion segments of the gaps of two neighboring arc extinguishing grid plates are staggered. According to the utility model, the resistance of an arc starting to entering into the arc extinguishing grid plates is reduced, the elongated distance of the arc is longer after the arc enters into the arc extinguishing grid plate, and the voltage of the arc rises rapidly, thus improving the short circuit disconnection capacity of the miniature circuit breaker.