The radial keyhole size is acquired by processing the coaxial image of keyhole captured in Nd∶YAG continuous wave (CW) laser deep penetration welding, and its dynamic characteristics are studied in this paper. The studied results show that the radial dimension of keyhole increases with laser power for partial penetration, while it is nearly unvaried when the workpiece is fully penetrated. With welding speed increasing, the radial width of keyhole monotonously decreases, but radial length of keyhole will be fluctuated. In certain range of defocusing distance, if laser intensity is high enough, the expansion of irradiated area by laser beam can make the radial keyhole size synchronously increase with laser spot.