Firmiana platanifolia is an important tree species for afforestation, but the lack of research on its response mechanism after water stress limits the development of its downstream industry. In order to explore the water tolerance degree and mechanism of Firmiana platanifolia, we analyzed the growth indexes of seedlings (plant height, stem diameter and leaf area), biomass allocation (leaf biomass ratio, stem biomass ratio, root biomass ratio, root biomass ratio and root biomass ratio) and photosynthetic index (net photosynthetic rate, transpiration rate, intercellular CO₂ concentration and stomatal conductance) under 30% field capacity (T1), 70% field capacity (T2), 100% field capacity (T3) and 130% field capacity (T4). The results showed that with the increase of water stress degree, all indexes of the seedlings firstly increased and then decreased, and the seedlings had best performance under T2 condition; the net photosynthetic rate was the highest under T3 condition, and the above-ground biomass allocation was 4% higher than that under T2 condition; under T2 condition, the leaf area and specific leaf area of the seedlings reached the maximum of 196.186 mm² and 512.48, respectively, and the total biomass accumulation reached the maximum of 17.14 g. T2 condition was the most suitable for the biomass accumulation of the seedlings, which could provide sufficient carbon reserve for the seedlings to successfully survive the winter; under T4 condition, the seedlings grew rapidly within 10 days and the growth trend weakened after 10 days, meanwhile, the aerial root emerged in response to flooding; T1 was the most unsuitable condition for the development of Firmiana platanifolia seedling. This study provides a theoretical basis for the responding mechanism of the seedlings under water stress.