Abstract: Flashing is a common two-phase flow phenomenon in open natural circulation systems. In order to investigate the effect of gas injection on the two-phase flow of open natural circulation flashing and enhance the circulation capacity and stability. In this paper, an experimental study was carried out to visualize the flashing flow characteristics of an open natural circulation system using water and air as media. The effects of different gas injection positions and volumes on the natural circulation capacity, flash two-phase flow characteristics, and resistance characteristics of the natural circulation system at different operating stages were investigated. The results of the study show that injecting air into the ascent section at different stages can effectively enhance the stability and circulation capacity of the natural circulation system. The effect of gas injection on the system is related to the location of the flash start point. Injecting gas upstream of the flash point has a better lifting effect than downstream, lifting the loop circulation flow by up to 8% to 10%. For the same injection position, injecting gas upstream of the flash point can enhance the flow capacity and stability of the circuit, injecting gas downstream has little effect on the flow of the circuit, but it does change the starting position of flashing. A mechanistic analysis and discussion of the effect of gas injection on the flashing flow is given, which demonstrates the effect on the loop flow due to the change in the loop pressure drop caused by the gas injection changing the share of vacuoles in the rising section of the loop. It is also concluded that the bubbles at the initial position of flash evaporation and the share of vacuoles introduced by gas injection near the onset of flashing contribute less to the loop, which provides a basis for two-phase flow modeling calculations.