Abstract: To reveal the effects of mixed planting density on tree growth and soil physicochemical properties in uneven-aged Cunninghamia lanceolata and Phoebe zhennan forests, Phoebe zhennan was interplanted in young (L1), medium-aged (L2), and mature (L3) Cunninghamia lanceolata stands, and tree growth and soil physicochemical properties were compared under three mixed planting densities: 2 m × 3 m (D1), 2 m × 4 m (D2), and 2 m × 5 m (D3). The results showed that the effects of mixed density on tree growth and soil physicochemical properties exhibited significant differences across forest age stages. During the young forest stage, Cunninghamia lanceolata and Phoebe zhennan grow better; during the middle forest stage, the D1 mode was more favorable for Cunninghamia lanceolata and Phoebe zhennan growth; and during the mature forest stage, the D2 mode showed better survival rate and growth performance for Phoebe zhennan. The mixed-species model generally improved the physicochemical properties of the soil, with the most significant effects observed during the sapling stage; however, the improvement of soil physical properties was not completely synchronized with growth performance. This study provides a theoretical basis for the precise transformation of Cunninghamia lanceolata plantations and the cultivation of Phoebe zhennan resources.