Abstract: This study aims to investigate the passivation behavior and mechanisms of low carbon steel in concrete pore solutions with different pH values. Through open circuit potential, linear polarization resistance, electrochemical impedance spectroscopy and Time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis, the effect of pH on the passivation behavior of low carbon steel was examined in detail. The results showed that in saturated calcium hydroxide (CH) solution with low pH, the polarization resistance (R_p) increased slowly and finally stabilized at 120 kΩ·cm², and the corrosion current density (I_corr) remained at a high level of 0.15 μA/cm². It shows that the CH solution has no obvious effect on the passivation of mild steel; While in the saturated Ca(OH)₂+0.1NaOH+0.3KOH (ST) solution with a higher pH, the capacitive reactance arc changes significantly within one day, indicating that it passivates within one day. Regardless of the pH value of the concrete simulation solution, mild steel can spontaneously form passivation film, and its surface passivation film went through the process from rapid initial growth to the later gradual stabilization. pH value is a key factor to promote the passivation of mild steel and there is a positive correlation with the performance of the passivation film, the thickness of the passivation film in the CH solution increased from the initial 1.930 nm to 3.733 nm, and the thickness of the passivation film in ST solution increased from 4.786 nm to 9.187 nm in one day in ST solution, indicating that the passivation film thickness tends to increase with the increase in pH of the simulated solution to form a more stable passivation film, which leads to the enhancement of its passivation performance.