We selected one-year-old seedlings of two broad-leaved trees, Koelreuteria paniculata and Schima superba, as study materials, potted under different concentrations of PbCl₂ solution (L1 < L2 < L3) in the greenhouse. We assessed the influence of Pb on the chlorophyll fluorescence characteristics in the leaves and revealed the function of photosystem Ⅱ (PSⅡ) under different Pb stress conditions in the contexts of energy balance and allocation, using the Lake-model. Our results can be used for the diagnosis of Pb resistance and rapid Pb stress in the seedlings of woody plants. The results showed that, under three different Pb treatments, increase in photosynthetically active radiation (PAR) for two tested cultivars increased the relative electron transport rate (rETR) and down-regulated the energy dissipation (Y_NPQ), but reduced the quantum efficiency of PSⅡ (Y_Ⅱ) and the photochemical quenching (qL). The non-light induced energy dissipation (Y_NO) did not change. At the same time, increased Pb concentration of the contaminant for two tested tree species reduced the maximum quantum use efficiency (Fv/Fm), rETR, Y_Ⅱ, and qL, but the Y_NPQ and Y_NO increased. The inhibitory effects of Pb on the chlorophyll fluorescence parameters of the two tested plant species were also reflected in the maximum net photosynthetic rate (Pn). In addition, this study also showed that, under the L1 gradient, the PSⅡ of S. superba remained relatively high under mild Pb stress, but suffered severe damage under high Pb stress, suggesting that S. superba has a much lower ability to convert light energy than K. paniculata. Y_NPQ greatly increased in K. paniculata and was more sensitive to Pb stress than S. superba, revealing strong Pb resistance due to a strong photo-protective mechanism in K. paniculata. Comprehensive analysis showed that Y_NPQ and Y_NO could be used as indicators for Pb stress diagnosis and evaluation of resistance to Pb stress in plants.