Phosphorous (P) is a crucial limiting nutrient for plant growth and development in terrestrial ecosystems. As one of the most important subtropical coniferous tree species, Chinese fir (Cunninghamia lanceolata) plays a major role in timber supply, climate mitigation and forest recreation. In this study, two Chinese fir clones (020 and 061) with a high growth rate were subjected to two levels of P supply in a greenhouse pot experiment: P-deficient P0 (no P supply) and P-replete P1 (1.0 mmol L−1 KH2PO4). Our objectives were to study the differences in morphology and growth patterns, cambial development mechanism and secondary root growth. Root sampling was performed at 40, 80 and 120 days after treatment initiation. Results indicated that the P-replete condition produced more root cambial cells (RCCs) in third class (larger) roots of clone-061 than in those of clone-020; concomitantly, clone-061 showed significantly greater stem height (42.87 ± 1.33 cm), root collar diameter (5.45 ± 0.126 mm), root average diameter (RAD, third class = 1.27 ± 0.04 mm) in different root classes and whole-plant biomass (7.55 ± 0.69 g) compared with clone-020 under the P-replete condition. Root diameter and biomass increase due to higher cambial activity resulted in greater seedling quality. In addition, a significant positive correlation was observed between the number of RCCs and root morphological traits (root length, root surface area, RAD and root volume). Root development was significantly promoted by cambium activity in the P-replete environment, thereby enhancing plant secondary growth. Based on these outcomes, we suggest that clone-061 would be more useful for enhancing production. Regarding commercial purposes, these findings will contribute to the improvement of P fertilization efficiency.