<p>Radon is a radioactive gas naturally present in red clay, and prolonged exposure to higher radon concentrations during engineering construction can increase carcinogenic risks and raise environmental pollution concerns. In this study, low-temperature nitrogen adsorption experiments and radon accumulation measurements were conducted to analyze the relationship between the pore structure parameters of red clay at different depths and the free radon production rate. The results indicate that the pore system of red clay is dominated by micropores and mesopores. With increasing depth, compaction leads to a reduction in macropores, a decrease in pore volume, and weakened pore connectivity, resulting in a negative correlation between the average adsorption pore diameter and depth. Free radon production rate exhibits an overall trend of “initial decrease followed by increase” with depth: shallow layers are dominated by diffusion and convection due to better pore connectivity; middle layers display a pronounced low point in free radon production rate due to denser pore structures; deep-level radon gas accumulation and a concentration gradient cause free radon production rate to rise. The study confirms that compaction induced by burial depth is the primary factor controlling free radon production rate in red clay, and the number of mesopores largely determines free radon production rate. These findings provide important scientific insights for environmental radiation risk assessment in engineering regions underlain by red clay.</p>