Immersion experiment was used to elucidate the corrosion mechanism of low‐alloy pipeline steel exposed to vapor‐saturated H2S/CO2 and H2S/CO2‐saturated brine conditions in terms of microstructure, corrosion kinetics, corrosion products, surface/cross‐sectional morphology, and elemental distribution. The experimental results demonstrate that the microstructure of designed steel is tempered martensite, and the corrosion rate in H2S/CO2‐saturated brine condition is larger than that in vapor‐saturated H2S/CO2 condition. The main corrosion product in two corrosion conditions is FeS, which shows various crystal structures. The size of corrosion products formed in vapor‐saturated H2S/CO2 condition is smaller than that of corrosion products formed in H2S/CO2‐saturated brine condition, which contributes to the difference of water chemistry in two corrosion conditions. Cr‐rich compounds are primarily formed on the coupons surface of two corrosion conditions. The schematic models of corrosion mechanism in vapor‐saturated H2S/CO2 condition and H2S/CO2‐saturated brine condition are proposed based on experimental results.