This study focused on sandstones from the Yungang Grottoes under different degrees of weathering and employed a specially designed experimental setup. Using a modified dew point temperature calculation formula, combined with measured temperature, humidity and electrical resistance data, a quantitative evaluation method was developed for water vapor diffusion and condensation processes in sandstone under varying degrees of weathering. Results showed that weathered sandstone had higher porosity and permeability, facilitating water vapor diffusion and condensation. During the water vapor diffusion stage, higher degrees of weathering were correlated with faster inward diffusion rates and greater internal relative humidity. During the condensation stage, as the degree of sandstone weathering increased, the location where condensation water formed became deeper and occurred earlier. In addition, the condensation depth in the rock was positively correlated with the duration of exposure to high-humidity environments. These findings can help in identifying the patterns of water vapor diffusion and condensation in weathered sandstone, thus providing a scientific basis for dealing with condensation issues at stone cultural heritage sites.