This paper presents a numerical modeling on the packing densification of uniform spheres under air impact using a combined Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM). In the whole process, the packing structure evolution, corresponding dynamics and densification mechanism are comprehensively studied. Macro and micro properties such as packing density, coordination number, radial distribution function, as well as forces in the packings at different stages are characterized and compared. The results show that air impact can realize the transition of particle packing from random loose to random close state at appropriate conditions. In this duration, the depth-averaged normal force increases linearly with the height. Meanwhile, numerous normal forces are close to the horizontal at the final packing stage due to the effect of air impact, and their distributions indicate exponential decay law. While the distribution of the fluid-particle interaction forces increases with the height. Local packing structure evolution demonstrates that the dominating densification mechanism under the air impact is 'pushing filling'. (C) 2017 Elsevier B.V. All rights reserved.