Electrochemical measurement and chemical immersion lest are employed to investigate the Inhibition mechanism of Na2MoO4 for carbon steel, which is studied in 55%LiBr+ 0.07mol/L LiOH solution at high temperature. Testing results indicate that the passive film on carbon steel surface is mainly composed of Fe304, and Mo is involved in the process of film forming, in a form of MoO3 and MoO2. The parametric analysis of the evolving potential and the blend potential of MoO2 shows that Mo mainly exists as Mo02 in passive film when the concentration of Na2MoO4 is lower than 150 mg/L. While its concentration is greater than 150 mg/L, it mainly exists as MoO3 inside the film and exists as MoO2 outside the film. MoO42-is deoxidized as MoO2 on the surface of carbon steel, which may impede the corrosion of active-site and raise the blend potential of carbon steel, and then MoO42-adsorbs onto the defects of the passive film and decompounds as Mo03 during the process of film forming. The electric field caused by different valence of Mo in passive film may retard the dissolution of carbon steel and lead to an increase in the polarization impedance and a decrease in hydrogen evolution. As a result, heavy concentrated Na2MoO4 solution (greater than 150 mg/L) has excellent Inhibition effect on carbon steel in LiBr solution.