In the combustion process, injection of secondary air is associated with achieving complete combustion and generating air pollutants. Building an effective secondary air system is important in combustion system design, and it is necessary to understand the reaction characteristics by secondary air injection. Several studies have been conducted on secondary air injection in combustion systems, but not many studies on the effect of injection method on combustion characteristics have been conducted. In this study, combustion and thermal flow characteristics according to various configurations of secondary air inlet in a burner using liquefied natural gas (LNG) as fuel were identified using computational fluid dynamics (CFD). The results show the effect of the recirculation area and flow change by the injection of secondary air on the temperature and CO production. This can be a reference for the effect of secondary air injection on combustion systems and can be used in various industries.