Abstract: A hydrophobic boron-dipyrromethene (BODIPY) derivative with aza and ammonium groups (BDP-NH₂) was hydrophilically modified by introducing quaternary ammonium groups. This chemical modification led to the synthesis of an amphiphilic quaternary ammonium-modified aza-BODIPY (BDP-QAS) molecule, which was designed to serve as a highly efficient small-molecule photothermal agent. By leveraging the interactions between hydrophobic and hydrophilic regions and utilizing ultrasonic self-assembly techniques, spherical nanoparticles were successfully obtained and named BD1. The spectral changes of both the small molecule and the assembled nanoparticles under 660 nm laser irradiation were thoroughly investigated by ultraviolet-visible（UV-Vis） spectroscopy. Results showed that there were notable differences in the spectral changes between the dispersed state and the aggregated state, thereby confirming the effective photothermal properties of BD1. The photothermal conversion efficiency of BD1 was meticulously calculated to be 45.1%, demonstrating its high efficiency. Moreover, this efficiency was found to remain largely unchanged even after five repeated cycles of heating and cooling, indicating excellent stability. Furthermore, the outstanding photothermal properties of BD1 were exploited in photothermal antibacterial experiments. These experiments demonstrated that BD1 exhibited significant bactericidal effects against both Staphylococcus aureus and methicillin-resistant Staphylococcus aureus (MRSA), showcasing its potential as a powerful photothermal antibacterial agent. Overall, the results highlight the promising applications of BD1 in photothermal therapy, particularly for antibacterial purposes, and suggest its potential use in medical treatments where effective bacterial eradication is required.