The invention relates to a circuit breaker and an arc extinguishing chamber thereof. The arc extinguishing chamber comprises a main nozzle and an expansion chamber communicating with the main nozzle.A gas storage chamber is arranged in the expansion chamber, and a damping structure communicating with the gas storage chamber and the expansion chamber is arranged on the cavity wall of the gas storage chamber. The pressure in the expansion chamber begins to rise at the initial stage of opening of the arc extinguishing chamber, and when the pressure in the expansion chamber is larger than that inthe gas storage chamber, gas in the expansion chamber enters the gas storage chamber through a damping hole to release partial pressure. When the opening is going to be finished, the pressure in theexpansion chamber is rapidly reduced, and when the pressure in the expansion chamber is smaller than the pressure in the gas storage chamber, gas in the gas storage chamber enters the expansion chamber through the damping hole, so it is guaranteed that the arc extinguishing chamber still has certain gas blowing capacity at the last stage of the opening, on-off of the circuit breaker is guaranteed,and the problems that in the prior art, the arc extinguishing chamber is insufficient in gas blowing capacity at the last stage of the opening, and on-off of the circuit breaker is not facilitated are solved.