Viburnum japonicum is an evergreen plant species, only found in the islands of Taizhou and Zhoushan, Zhejiang Province in China. This species was listed in the Directory of Plant Species with Extremely Small Populations in Zhejiang Province, because the wild individuals were extremely rare. To study its resistance mechanism and adaptation to drought, reveal the cause endangering the species, 2-year-old seedlings of V. japonicum were potted and grown under four soil water conditions:normal water supply (CK, 80% of the field moisture capacity), mild water stress (LS, 60% of the field moisture capacity), moderate water stress (MS, 40% of the field moisture capacity), and severe water stress (SS, 20% of the field moisture capacity). Determinations were implemented on photosynthetic characteristics in the 15, 30, 45, 60, and 75d of treated seedlings. The result demonstrated that V. japonicum seedlings could grow normally under CK, LS, and MS, but they died at 75 d under SS. The net photosynthetic rate (P_n), SPAD value, maximum net photosynthetic rate (P_nmax), apparent quantum yield (AQY), and dark respiration rate (R_d) of different treated seedling leaves decreased with the decrease of soil water content, while the light saturation point (LSP) and light compensation point (LCP) increased. The AQY increased gradually over time under all moisture gradients, and P_n was below 0μmol m^-2 s^-1 from 45d under SS. The changes in the maximum quantum yield of photosystem Ⅱ (PSⅡ) (F_v/F_m) was small during any stress; the actual photochemical efficiency of PSⅡ (φ_PSⅡ), electron transport rate (ETR) and photochemical quenching coefficient (qP) of seedlings decreased with increasing water stress, but the non-photochemical quenching coefficient (NPQ) showed the opposite trend. The differences in the chlorophyll fluorescence parameters changed slightly among LS, MS, and CK, while the gaps between SS and CK increased. The experiment resulted that V. japonicum seedlings under LS and MS showed some degree of adaptability and resistance to drought, but be seriously influenced under SS with long duration.