This paper focused on the mixer optimization by numerical simulation. The mixing and flow characteristics inside two different lobed mixers with/without centrum were obtained by three-dimensional CFD simulation. The core flow was the hot rich-methane/O2（l）burnt gas, while the cold air flew by the by-pass. The air/burnt gas flow ratio was improved from 7 to 9 in order to confirm the effect of air/burnt gas ratio on the mixing and flow characteristics. The simulation results indicated that no matter which mixer was used, there were a pair of symmetrical recirculation regions in the mixers, and the total temperature and species distribution turned to be more uniform at the increased mixing length. The mixing performance in the lobed mixer with centrum was slightly better than that of the lobed mixer without centrum, and the length of recirculation region in the lobed mixer with centrum was slightly shorter than that of the lobed mixer without centrum. The air/burnt gas ratio had considerable effect on the mixing and flow characteristics. The mixing performance with air/burnt gas ratio of 9 was much better than that of air/burnt gas ratio 7. Similar mixing performance would attain in the case of air/burnt gas ratio 9 with only half of the mixing length in the case ofair/burnt gas ratio 7. The recirculation region in the case of air/burnt gas ratio 9 occurred ahead of that of air/burnt gas ratio 7, and the former was longer than the latter.