Immersion experiments and electrochemical measurements were carried out to study the effect of tempered martensite and ferrite/bainite on corrosion behavior of low alloy steel used for flexible pipe exposed to high-temperature brine environment in terms of microstructure morphology, corrosion rate, corrosion products characteristics, potentiodynamic polarization curves and electrochemical impedance spectroscopy. The experimental results indicated that the proportion of high-angle grain boundaries of tempered martensite steel was greater than ferrite/bainite steel. The corrosion rate of ferrite/bainite steel was lower than tempered martensite steel. With increasing immersion time, the structure of corrosion products formed on the surface of ferrite/bainite steel was more compact and denser than tempered martensite steel. In addition, the current density of ferrite/bainite steel was lower than tempered martensite steel. The charge transfer resistance and total impedance of ferrite/bainite steel were larger than tempered martensite steel. Thus, the corrosion resistance of ferrite/bainite steel exposed to high-temperature brine environment was excellent compared to the tempered martensite steel.