The studies of UV illumination and ammonium salt on the atmospheric corrosion of steel were rare. The combined effect of UV illumination and NH4Cl salt on the atmospheric corrosion of Q450 weathering steel was explored in this work. Mass changes results showed UV illumination distinctly accelerated the atmospheric corrosion of Q450 weathering steel depositing with NH4Cl. The corrosion products with semi‐conductor properties could improve the corrosion current and decrease the resistance of rusted samples when introducing light illumination during pre‐Tafel test. Mott‐Schottky analysis, photoinduced current density, and photoinduced open‐circuit potential change illustrated that the electric conductivity of corrosion products played a big role in the accelerated corrosion process.