This work investigated the corrosion characteristics of FeCrAl alloy in supercritical water (SCW), and focused on the effect of SCW pre-corrosion on the alloy oxidation and nitridation at 900 °C. FeCrAl alloy formed outer Fe-rich oxides (mainly Fe2O3) and inner Fe-Cr-Al oxides during SCW exposure. SCW pre-corrosion made FeCrAl alloy exhibit different morphologies and phase compositions in the same medium. SCW pre-corrosion promoted the deposition of Ni and Mo elements from the autoclave, filling the gaps in flaky oxides by forming Ni- and/or Mo-rich products in air and N2. Moreover, SCW pre-corrosion avoided the appearance of accelerated nitridation zones in N2. However, pre-corrosion caused the preferential diffusion of N2 and air. Outward-diffused Cr ions during SCW exposure reacted with O impurities in N2 to form Cr2O3 that easily dissociated N2, leading to the development of nitridation towards the alloy interior.