Abstract: This study employed minirhizotron imaging to investigate the fine root morphological characteristics of 33 individuals of Cupressus gigantea W. C. Cheng & L. K. Fu across a gradient of diameter classes, determined by diameter at breast height (DBH) and age, under in situ conditions. Concurrently, key soil environmental parameters were assessed to identify the regulatory factors influencing fine root morphology. Results revealed significant differences in fine root traits across diameter classes, with larger trees exhibiting greater total fine root length, surface area, and volume. In addition to tree size, soil physicochemical properties also exerted a pronounced influence on fine root morphology. Structural equation modeling indicated that the overall impact of soil conditions (total effect=0.98) surpassed that of tree diameter (total effect=0.23). Among the measured variables, soil bulk density, moisture content, electrical conductivity, and oxidation-reduction potential emerged as principal drivers, acting either directly or indirectly by modulating soil nutrient availability. These findings suggest that fine root morphological characteristics in C. gigantea are jointly regulated by diameter and soil physicochemical properties, with the latter playing a dominant role.