This study primarily explored the limiting factor for photosynthesis during the development of Euonymus japonicus leaves. The analysis of the chlorophyll fluorescence transient, pulse-modulated fluorescence, 820-nm reflection, and expression of core proteins for photosystems demonstrated that photosystem II (PSII) activity developed more rapidly than did photosystem I (PSI) activity. The slower development of the PSI activity restricted linear and cyclic electron transport and thus inhibited the production of ATP and NADPH, which inhibits the activation of Rubisco, resulting in low activity of carboxylation efficiency. The application of exogenous NADPH (50 μM) and ATP (100 μM) to leaves remarkably increased the Pn and CE in the youngest leaf but not in the fully expanded leaf, which indicated that an inadequate supply of the assimilatory power significantly inhibited CE and Pn. We concluded that the slower development of the PSI activity was one of the most important limiting factors for photosynthesis during the development of E. japonicus leaves.