The mechanisms are different for intense pulsed ion beam ablation plasma expanding into ambient gas of different pressure or into vacuum. The former is complicated by the interaction between plasma and the ambient gas. The 2D gas dynamical model has been built to describe the process, and the plasma expansion into ambient gas of pressures from 10-6 atmosphere to 1 atmosphere have been calculated. The results show that the phenomena of plasma expanding into ambient gases vary as the pressure changes. It can expand into vacuum freely, but into atmosphere its movement is confined. And the ejection into ambient gas of one thousandth atmosphere or so is special, the ‘snowplow’ phenomenon is observed and the plume plasma splits into fast- and slow-moving parts.