Abstract: To elucidate of the optical adaptation mechanism of Sinojackia dolichocarpa under different light intensities,and to provide theoretical implications for the reproduction and ex situ conservation of S.dolichocarpa populations,two-year-old seedlings of S.dolichocarpa were used as the research objects and treated with four light intensities of 100% full light(CK),60% light (L₁),40% light (L₂) and 20% light (L₃). The photosynthetic characteristics of S.dolichocarpa seedlings at different light intensities were measured and analyzed. The results were presented below. Under CK and L₁ conditions,the leaves of S.dolichocarpa seedlings showed a "midday rest" phenomenon of photosynthesis in summer. Under L₂ and L₃ conditions,the daily variation of the net photosynthetic rate (P_n) of S.dolichocarpa seedlings showed a single peak curve. The daily means of net photosynthetic rate (P_n),stomatal conductance (G_s) and transpiration rate (T_r) under L₃ condition were significantly higher than those under CK and L₁ conditions(P<0.05). The maximum net photosynthetic rate (P_nmax),light saturation point (LSP),maximum carboxylation rate (V_cmax),maximum electron transfer rate (J_max),and trisaccharide phosphate utilization rate (TPU) of S.dolichocarpa seedlings under L₃ condition were significantly higher than the other three conditions(P<0.05). The light compensation point (LCP) and CO₂ compensation point (CCP) under CK conditions were significantly higher than those under L₃ condition (P<0.05). The initial light energy conversion efficiency of PSII(F_v/F_m),the actual light quantum yield of PSII(F'_v/F'_m),and the electron transfer rate (ETR) of S.dolichocarpa seedlings were all highest under L₃ condition. In a word,under full sunlight in summer,the seedlings of S.dolichocarpa showed a "midday rest" phenomenon in photosynthesis with a decrease in carbon assimilation efficiency,showing significant light inhibition. L₃ condition was suitable for the growth of young seedlings of S.dolichocarpa with high P_n and strong carbon assimilation ability. The young seedlings of S.dolichocarpa exhibit the characteristics of shade plants.