Abstract Flash boiling can enhance fuel breakup and atomization via the quick growth and eruption of bubbles when the high temperature fuel exiting the nozzle. For multi-hole fuel injectors, a high level of flash boiling might cause the plumes to merge into a single plume. Such phenomenon is known as spray collapse, which will change the preset spray targeting in real engines, and is not desirable for practical application due to a longer penetration it might cause. Although extensive works have been done on the collapse of flash-boiling, no consensus has been reached for its core mechanisms yet. This work used a two-hole injector to study the flash-boiling plume-to-plume interaction, which is considered to be a key inducement of spray collapse. Optical measurement approaches were used to examine the influence of fuel temperature, ambient pressure, and injection pressure on the morphology of the flash boiling spray. It was found that a secondary plume was generated by two primary plumes side-collision in the spray central region under some flash-boiling conditions, and its behavior had a clear relation with the level of flash-boiling and other boundary conditions. Additionally, elevating injection pressure and increasing ambient pressure are two doable options for controlling the flash-boiling targeting in real engines, through which non-collapse flash-boiling sprays can be obtained. Finally, the relation between the secondary plume and the spray collapse was clearly demonstrated, and possible collapse mechanism was discussed in details.