Physical mechanisms of gas recirculation and wake closure were investigated by modeling the gas field generated by High Pressure Gas Atomizer using computational fluid dynamics. A recirculation mechanism based on axial and radial gas pressure gradient was proposed to explain the gas recirculation. The occurrence of wake closure is regarded as a natural result when elongated wake is gradually squeezed by expansion waves of increasing intensity. An abrupt drop could be observed in the numerical aspiration pressure curve, which corresponds well with the experimental results. The axial gradient of gas density is considered as the reason that results in the sudden decrease in aspiration pressure when wake closure occurs. Lastly, it is found that a shorter protrusion length and a smaller melt tip diameter would lead to a smaller wake closure pressure, which could benefit the atomizer design to produce fine metal powder with less gas consumption.