Abstract: Three problems of the coaxial output cavity operating at TM310 mode for Multiple-Beam Klystron (MBK) are concerned: the field distribution symmetry, the external Q-factor of operating mode, and the frequency interval between operating mode and neighboring modes. In order to improve the performance of coaxial output cavity, a cavity with reentrant ring and short circuit pole is proposed. Setting the short circuit pole suitably can not only decrease external Q-factor, but also make field distribution uniform. Furthermore, inserting the reentrant ring can enlarge frequency interval between operating mode and neighboring modes to suppress the parasitic modes. The coaxial output cavity in X-band is modeled and simulated by CST-MWS. And then the actual output cavity is manufactured and tested. The tested frequency intervals between TM310 mode and neighboring modes are both enlarged to more than 1.2 GHz. The external Q-factor is diminished from 507.4 to 193.5. And the symmetry of the field distribution of TM310 mode is improved.