Seawater pumped storage systems have bright prospect for energy storage in the coming years. The operational conditions of seawater pumped storage system are complex and harsh, where metal materials suffer from severe general and local corrosion. The corrosion behavior of Q235B carbon steel in simulated seawater pumped storage system under operational conditions was studied by potentiodynamic polarization, cyclic potentiodynamic polarization, and scanning electron microscope (SEM). The results confirm that the working pressure affected the corrosion resistance of Q235B carbon steel during the whole immersion period. The pressure promoted the electrochemical reaction of corrosion process and the corrosion rate increased with pressure at the initial immersion period. However, the stable rust layer formed after long-time immersion at different pressures increased the corrosion resistance of carbon steel, and decreased the corrosion degree of carbon steel. Meanwhile, the working pressure affected the pitting corrosion behavior of Q235B carbon steel during the whole immersion period. The pitting corrosion potential was more negative and the tendency of pitting corrosion was higher at 4 MPa during the whole immersion period. However, pressure also accelerated the formation rate of protective rust layer on the steel surface. Q235B carbon steel has higher susceptibility to pitting corrosion at 4 MPa in the static seawater.