The visual experiment of subcooled flow boiling is performed in a vertical square channel with one heating rod for water. The objective is to investigate the vapor-liquid interface fluctuation characteristics to reveal the trigger mechanism of subcooled flow boiling critical heat flux (CHF), when the phenomenon of CHF occurs. The test is conducted with mass fluxes of 800 to 1200 kg/(m2s) and inlet subcooling between 40∼60 K. Besides, the system pressure is 135 kPa. The vapor-liquid interface behavior under near CHF conditions is captured downstream of the test section by a high-speed camera with 4000 fps. Based on experimental observations, a double wavy liquid-vapor interface propagates along the heating wall due to Helmholtz instability. One is the vapor layer and liquid film near the wall, and the other is the vapor layer and outer liquid core. The parametric effect of vapor-liquid interface fluctuation characteristics has been explored quantitatively and qualitatively with variations in inlet subcooling in detail. The analysis finds that the wavelength of the vapor layer and liquid film decrease with inlet subcooling increases. It mainly resulted from that interface condensation increases with the inlet subcooling increasing. Furthermore, the wavelength of liquid film varies approximately linearly with inlet subcooling. An empirical correlation is developed based on the experimental measurement value. This study will contribute to further developing the trigger mechanism of CHF.