The volume of the arc extinguishing chamber of the generator circuit breakers (GCBs) determines the total amount of SF6 gas in the arc extinguishing chamber, thereby affecting its pressure building and arc blowing effects. Ultimately, it affects the breaking capacity of the GCBs. This paper proposes three different radius arc extinguishing chamber structures for the size of the arc extinguishing chamber volume. The arcing characteristics of different arc extinguishing chambers are obtained through magneto hydro dynamics (MHD) simulation. The results show that if the volume of the arc extinguishing chamber is too small, high pressure can be established in a short period of time, which is conducive to the formation of strong gas blowing. But the stored gas is limited, which is not conducive to extinguishing the arc. If the volume of the arc extinguishing chamber is too large, there will be more gas stored. But the gas pressure in the arc extinguishing chamber is relatively low, which is not conducive to the formation of strong gas blowing. Comparing the maximum temperature on the symmetry axis of the three structures, the arc extinguishing chamber with R = 80 mm is 7.09% and 17.59% lower than the arc extinguishing chamber with R = 72 mm and R = 88 mm, respectively. It shows that there is an optimal value for the volume of the arc extinguishing chamber. This study can provide reference for improving the breaking performance of the GCBs.