Abstract: The influence of NaCl solution temperature (35-55℃) and relative humidity (60%-80%) on corrosion resistance of Q345 steel under simulated high temperature and humidity marine atmosphere was investigated by wet/dry cyclic test, scanning electron microscopy, X-ray diffraction, etc. The results show that with the increase of solution temperature, the corrosion rate of tested steel increased and had a relatively high increasing rate at a relatively high temperature; the thickness of the surface rust layer increased and the hole inside the layer became larger; the content of the Fe₃O₄ corrosion product also increased. With the increase of relative humidity, the corrosion rate of the tested steel increased, the rust layer became thicker and thicker while the phase composition of the rust layer changed very little.