Understanding the variation and correlation of plant functional traits among different planting patterns is of great importance to reveal the ecological adaptability and function of different tree species. The variations of functional traits in two common tree species' seedlings (Cunninghamia lanceolata and Zelkova schneideriana) in subtropical area were studied among different mixing proportions based on pot experiment. These mixing proportions included a single-species C. lanceolata planting pattern (4C), a single-species Z. schneideriana planting pattern (4Z) and three mixed treatments (1C3Z, 2C2Z, and 3C1Z). The results showed that:(1) total leaf area, leaf dry matter content, net photosynthetic rate, transpiration rate and stomatal conductance of C. lanceolata decreased significantly in the mixed treatments compared to 4C treatment, while specific leaf area increased significantly. Root length and specific root length were no difference among different treatments. Leaf biomass, stem biomass, root biomass and total biomass per plant of C. lanceolata among different treatments were significantly lower than in the 4C treatment, and these variables were no significant difference among mixed treatments. (2) mean leaf area of Z. schneideriana was the highest in the 3C1Z treatment. Total leaf area increased gradually with the decrease of the proportion of Z. schneideriana in tree species composition, specific leaf area was no significant difference among treatments, leaf dry matter content was the highest in the 2C2Z treatment. Net photosynthetic rate, transpiration rate and stomatal conductance of Z. schneideriana were the highest in the 2C2Z treatment, while the instantaneous water use efficiency was the lowest in the 2C2Z treatment. Root length of Z. schneideriana increased significantly in the 3C1Z treatment, while specific root length showed less variation among different treatments. Leaf biomass, stem biomass, root biomass and total biomass per plant of Z. schneideriana increased gradually with the decrease of the proportion of Z. schneideriana in tree species composition. Overall, interspecific competition was greater than intraspecific competition for C. lanceolata in mixed C. lanceolata and Z.schneideriana treatments, while the pattern was opposite for Z. schneideriana. With the decrease of the proportion of C. lanceolata in mixed treatments, C. lanceolata enhanced interspecific competition by increasing specific leaf area and net photosynthetic rate and reducing stem biomass accumulation. While with the decrease of the proportion of Z. schneideriana in mixed treatments, Z. schneideriana significantly increased leaf area and root length to improve resource utilization, and reduced underground resource allocation and increased aboveground stem biomass accumulation. Therefore, mixing proportion of tree species will significantly affect functional traits and biomass accumulation. Determining appropriate mixing proportion is of great significance for sustainable management of mixed-plantation.