The main objective of this study is to investigate the effect of ambient pressure (P-amb) on the morphology of multi-jet flash boiling sprays and to promote the understanding on the collapse mechanism. With the high-speed imaging technique, the n-hexane spray process in a constant volume vessel was captured under the ambient pressures from 2 kPa to 101.3 kPa and the fuel temperatures (T-fuel) from 20 degrees C to 120 degrees C. The spray width within the axial distances of 20 nozzle diameters (do) could be increased by increasing T-fuel and/or decreasing P-amb. Beyond 20do, the change in spray width against superheated degree (Delta T-SD) was fairly sensitive to P-amb. It decreased with the increase in Delta T-SD at P-amb larger than 50 kPa, and increased with Delta T-SD at P-amb less than 50 kPa. Based on the new understanding that the collapse in multi-jet flash-boiling sprays was induced by vapor condensation at the nozzle exit and the resultant local pressure drop (i.e. generation of low-pressure zone), the sensitivity of spray width on P-amb could be well explained as follows: relatively larger P-amb (> 50 kPa for the used injector) would be conducive to the vapor coagulation and cause stronger vapor condensation (i.e. larger pressure drop) while the vapor dispersion may become dominant at lower P-amb (< 50 kPa for the used injector), negatively influencing the intensity of vapor condensation.