Flash boiling spray can increase fuel evaporation and spray atomization efficiency effectively despite under the low injection pressure. However, in order to study the detailed influences of fuel injection pressure on the breakup process of flash boiling sprays and related mechanisms, the highspeed imaging experiments under different fuel injection pressures and overheat degrees were conducted by using the shadowgraph method on a single-hole fuel injector. Then the impacts of injection pressure and overheat degree on spray morphology and spray breakup were compared and the breakup mechanisms were also analyzed under different conditions. The research shows that the width of spray increases obviously and the penetration shortens remarkably after increasing the overheat degree of fuel. Besides, the spray penetration decreases and the spray width increases unobviously in the cooling spray and slightly increases and obviously reduces in the overheating spray.