The invention relates to a system and method for measuring instantaneous gas density for conversion from long-air-gap streamer discharge to leader discharge. The system includes an LED chip, a bean expanding lens, a main lens, a convergent lens, a knife edge, a telephoto lens and an ICCD camera. A discharge zone is generated between the main lens and the convergent lens. A coupling lens focuses self-luminescence generated by discharging and transmits the self-luminescence into a photomultiplier. The photomultiplier converts optical intensity time domain signals of the discharge self-luminescence into a voltage waveform and inputs the voltage waveform into a TTL pulse formation circuit. An output end of the TTL pulse formation circuit is connected to an input end of a digital delay generator, and the output end of the digital delay generator is connected to a pulse drive power supply and the ICCD camera separately. The system and the method can achieve accurate inversion measurement ofan ns-stage instantaneous gas density distribution rule in a process of conversion from long-air-gap streamer discharge to leader discharge, and have advantages of high system reliability and a high capturing efficiency.