Compared to water jet,supercritical carbon dioxide (co₂) jet can break rocks more efficiently at lower pressure,without causing permeability damage on reservoir. Its potential applications in radial drilling and well stimulation for unconventional resources have been attracted much attention. The unique properties of supercritical co₂ fluid as well as the jet impingement and rock-breaking performance can be affected by ambient pressure,especially under complex bottom-hole conditions. Corresponding numerical simulations and lab experiments were carried out. The results show that,at constant inlet jet pressure,the jet impinging pressure and depth of eroded hole both decrease notably with the increase of ambient pressure. Under constant pressure difference,as the ambient pressure increases,the jet impinging pressure almost keeps no change,while the eroded depth increases slightly,then tends to decrease at a moderate rate and is bounded by the critical pressure of co₂. The different rock-breaking performance against water jet is attributed to the unique fluid properties of supercritical co₂. Increased ambient pressure makes co₂ change from gaseous to supercritical phase,and causes a non-submerged supercritical co₂ jet to a submerged one.