Keyhole tungsten inert gas (K-TIG) welding is a variant of TIG welding, which can largely improve the weld penetration depth by forming keyholes inside the molten pool during welding. However, K-TIG welding is generally considered unsuitable for aluminum alloys due to their high thermal conductivity. A novel double-pulsed variable polarity TIG (DP-VPTIG) welding process was employed and the stable full penetration keyhole welding of 7 mm-thick AA2219 aluminum alloy was achieved. Keyhole dynamic evolution for DP-VPTIG was investigated based on visual sensing technology. Results indicate that in low-pulsed peak stage of DP-VPTIG process, the keyhole forms under the dominant role of the downward arc pressure against the upward surface tension and hydrostatic pressure acting on the surface of the molten pool, while the keyhole is closed as the upward surface tension and hydrostatic pressure become the dominant role in low-pulsed base stage. The periodic variation of low-frequency pulse in DP-VPTIG process stimulates a periodic keyhole behavior of “opening” and “closing” in the molten pool. The formation of keyhole is beneficial to the increase of weld penetration depth as the arc moves downwards along the keyhole and directly heats the solid metal under the molten pool. The keyhole size decreases with the increase of low-pulsed frequency.