In comparison with single-component fuel spray, the atomization and evaporation of multi-component fuel spray is more complex due to the physical property difference among the fuel components. This complexity is more significant when the fuel spray is formed under flash boiling condition. Therefore, the investigation of atomization and evaporation of multi-component fuel has recently become a practical research focus. However, due to the lack of related experiments and theoretical explanations, the spray formation under flash boiling condition has yet to be fully revealed. To address such challenges, this study is focused on high-speed optical investigation of multi-component flash boiling spray phenomena. Test fluids were gasoline surrogate fuels composed of three components (n-pentane, iso-octane, and n-decane). A scaled-up transparent two-dimensional nozzle was fabricated for the optical investigation. The droplet diameter was also measured using Phase Doppler Interferometry (PDI). Experimental results show that mixing of high volatility fuel with low volatility fuel can significantly enhance the generation of flash-boiling bubbles in the internal flow inside the nozzle (in-nozzle flow). Meanwhile, the increase of flash boiling bubbles can promote fuel breakup in the near-nozzle region, resulting in fuel droplets of smaller SMD with more uniform distributions. Besides, proper mixing ratio of high volatility fuel and low volatility fuel can moderate the layered structure (bubble and liquid flows) inside the nozzle and therefore can stabilize the breakup process from liquid ligaments into fuel spray with low variation of spray geometry.