With the exploitation of high pressure gas fields and the development of carbon capture and storage (CCS) techniques, the corrosion problem of steels under CO₂ environment has been paid more and more attention. To transportation easier and cost reduction, CO₂ in pipelines and containers is usually pressured to a high pressure, such as supercritical state. The supercritical CO₂ corrosion environment includes the CO₂-saturated aqueous phase and the water-saturated supercritical CO₂ (SC CO₂) phase. Moreover, corrosive ions such as Cl^- usually exist in CO₂ corrosion environment, which could accelerate the occurrence of corrosion. Low alloy steels, widely used as pipelines and construction materials in oil/gas and CCS industries, are susceptible to corrosion in the aggressive environment that contains high-concentration ions and acidic gases, especially to severe localized corrosion. In this work, the crevice corrosion behavior of 3Cr and X70 steels exposed in supercritical CO₂-saturated 3.5%NaCl solution and NaCl solution-saturated supercritical CO₂ phase was investigated. SEM, EDS and 3D laser microscopy were used to analyze the corrosion product scale on the steel surface. The results show that both the steels occurred crevice corrosion on the edge of crevice, but slightly occurred corrosion inside the crevice. The crevice corrosion occurred due to the galvanic effect of areas inside and outside the crevice. In supercritical CO₂ phase, 3Cr steel exhibited a higher uniform corrosion resistance than X70 steel, while the crevice corrosion resistance of 3Cr steel was lower than that of X70 steel. The different crevice corrosion behaviors between X70 and 3Cr steels might be attributed to the synergistic effect of elements Cr and Cu on enhancing the crevice corrosion.