Abstract: The effects of no light, half-day light and all-day light on the corrosion behavior of Q450NQR1 weathering steel in the wet-dry cycle condition were examined by corrosion mass loss, scanning electron microscopy, X-ray diffraction and linear polarization tests. The results indicate that light significantly affects the corrosion process in the wet-dry cycles. In the early stage of steel corrosion (before 48h), the corrosion rate from fast to slow is followed by half-day light, no light, and all-day light; but in the later period (72-96 h), the corrosion rate is ordered by half-day light, all-day light, and no light. The corrosion morphologies are obviously different. The corrosion products are silt-shape distribution in no light condition and particulate distribution in light condition, while they are particle-distributed with finer particles and smaller pores in all-day light condition. X-ray diffraction results show that the corrosion products are the same, but the contents are different in the three conditions. By linear polarization tests, the polarization resistance is obtained in the order from fast to slow of no light, half-day light, and all-day light at 96 h, which is well consistent with the order of corrosion mass loss rate.