This study aimed to investigate the growth and physiological characters of Liriodendron tulipifera seedlings in responses to two different levels of elevated air temperature and $CO_2$ concentration. The seedlings were grown in environment-controlled growth chambers with two combinations of air temperature and $CO_2$ conditions: (1) $22^{circ}C$ + ambient $CO_2$ $380{mu}mol;mol^{-1}$ and (2) $27^{circ}C$ + $770{mu}mol;mol^{-1}$. Physiological characters such as growth, photosynthesis, and water use efficiency, were monitored for 85 days. The seedlings under the elevated treatment showed a greater amount of growth in tree height, compared with those under the control. Regarding the characteristics of assimilatory organs, the elevated treatment resulted in a greater amount of total leaf area, leaf unfolding, and dry weight per leaf area. No significant differences were found in photosynthesis capacity between the two treatments. The increase in water use efficiency with increased intercellular $CO_2$ partial pressure appeared overall lower in the seedling under the elevated treatment, compared with those under the control. The total leaf area of the seedlings under the elevated treatment was larger than that under the control, indicating a higher amount of photosynthesis. In addition, an increase of root growth was noted under the elevated treatment. A resistance mechanism of water stress may be attributed to a higher amount of organ growth as well as the tree height under the elevated treatment than the control.