In order to improve the interrupting capacity of MCCB (molded case circuit breaker), the arc chamber of MCCB is optimized.Based on the theoretical study of the arc breaking process,three methods were used to improve the arc voltage.Firstly,the structures of arc plates were optimized,the pattern of the first piece's guide,the last piece's obstruction and the middle pieces' normal arrangement was used to achieve a good effect of arc cutting.Secondly,the mag-netic flux structure of slot motor was added,the speed of arc could be increased by increasing the intensity of magnetic field,the results of ANSYS simulation showed that the new structure in-creased 97 N magnetic blow force.Thirdly,air blowing structure was added,the voltage drop of arc column was increased by reducing the temperature of arc column,and PA66 was used as the gassing insert which had the fast recovery rate of medium.The methods were combined to get the optimized design scheme and to make the prototype sample to do short circuit interrupting test under the condition of 25℃ & 415 V AC voltage.Originally,it could only pass 50 kA,after op-timization it could pass 70 kA and the arcing time was shortened from 5 ms to 3 ms at the same time.It could be found that some of the products appeared arc chamber shell cracked issue after disassembling these samples,so the wall thickness of the crack position was increased 2 mm,the static analysis module of ANSYS was used to compare the equivalent stress of the shell before and after the change.The results showed that the new structure improved the strength of 40.2%, and it passed the engineering test after the new prototype samples were assembled again.The re-searching achievement has a certain significance for the follow -up new development and design improve-ment of MCCB products.