Abstract: The rapid chloride migration coefficient (RCM) method and the natural immersion method were used to study the chloride ion penetration resistance of multi-walled carbon nanotubes (MWCNTs) reinforced concrete. The chloride ion diffusion depth on the longitudinal section of the concrete specimen was measured, and the chloride ion diffusion coefficient was calculated based on this. The test results show that when the content of MWCNTs is 0.15wt%, the chloride ion diffusion depth and chloride ion diffusion coefficient of concrete at 28 days are reduced by 25.7% and 19.1%, respectively. Under four different erosion ages of natural soaking, mixing in the concrete of MWCNTs, the internal chloride ion concentration is always lower than that of the control group. Combining the analysis of the two methods, it is concluded that the free chloride ion concentration at each depth of the concrete decreases with the increase of the MWCNTs content, so that the chloride diffusion coefficient decreases with the increase of the MWCNTs content, and the incorporation of MWCNTs improves the resistance of concrete to chloride ion penetration. In addition, through SEM and mercury intrusion (MIP) tests, the microscopic enhancement mechanism of MWCNTs on the anti-chloride ion permeability of concrete was further explored. The analysis results show that MWCNTs have a certain bridging and filling effect, which may cause concrete crack propagation to be affected, inhibit and refine the pores, thereby improving the microstructure of concrete and improving the resistance of concrete to chloride ion penetration.