Experiments were carried out in this paper first to clarify the characteristics of a digging process during aluminum alloy of type 5A06 variable polarity plasma arc welding (VPPAW). A special "Blasting type" penetrating phenomenon was observed through measuring the penetrating time by electrical and optical signals. A 3D model was established to numerically investigate heat transfer, fluid flow and surface shape during the digging process. An adaptive heat source was adopted to describe heat transfer process involving deformation of the keyhole tracked by volume-of-fluid (VOF) method. The plasma arc pressure attenuated in a parabolic form as the depth of the keyhole increases. It is found that the weld pool surface becomes concave after sufficient melting of metal. The keyhole depth exhibits responsive changes, which corresponds with the different stages of welding parameters. In the late stage, the depth increase rapidly to fully penetrating state, which shows a "blasting type" penetrating process. The keyhole formation was interpreted by considering energy accumulation and mass conservation of fluid flow. The maximum velocity during digging process always occurs on the keyhole surface, where a vortex in the opposite direction appears inside the weld pool. The calculated penetrating time, keyhole size and fusion line were basically in agreement with the experimental results. (C) 2017 Elsevier Ltd. All rights reserved.