The resistivity is an important characterization method for the solidification of cementitious materials. To investigate the effect of CaO addition on the electrical properties of red mud, resistivity tests were conducted on red mud samples with different CaO contents. Low-field nuclear magnetic resonance, X-ray diffraction, and electron microscopy were used to reveal the mechanism of resistivity changes in the samples. The results show that the resistivity of the samples gradually increase with the curing age, and the peak resistivity corresponding to different ages have different CaO contents. With the increase of the CaO content and curing age, the porosity and pore size of the samples increase, the connectivity of pore water decrease, resulting in a decrease in the conductive pathways and an increase in resistivity. At the same time, the conductivity of pore water gradually increase, indicating that the impact of pore water connectivity on resistivity is higher than that of pore water ion concentration. The results can provide a theoretical basis for non-destructive testing of related structures.