Abstract: On the basis of experiments at atmospheric pressure, a three-dimensional simulation study of the spray pattern and breakup mode of gas-centered swirl coaxial (GCSC) injector was conducted with the RNG k-ε model. The gas-liquid interface was captured by the adaptive mesh refinement (AMR) technique, coupled level-set and volume of fluid (CLSVOF) method. The results demonstrated that when the liquid mass flow rate (\dotm_\mathrml) is constant, the ejection action of the central airflow is enhanced along with the gas mass flow rate (\dotm_\mathrmg). The internal and external pressure difference of the liquid film increases and the spray angle decreases. Meanwhile, the flow characteristics were analyzed. When \dotm_\mathrmg is constant, the radial velocity and tangential velocity of the liquid film at the injector outlet increase with \dotm_\mathrml, which may result in the increase of the spray angle. According to the gas-liquid mass flow rate (GLR), the breakup modes of spray are divided into the perforation breakup, bubble breakup and pneumatic breakup.