The effects of salt stress (100 mmol L-1 , 200 mmol L-1 NaCl) on photosynthetic characteristics and photosystem Ⅱ (PSⅡ) activity in sweet sorghum seedlings grown outdoors were carefully investigated in this study.Results showed that the Na+ content and the ratio of Na+/K+ in leaves increased significantly with increase of NaCl concentration.The net photosynthetic rate (Pn), the efficiency of excitation energy captured by open PS Ⅱ reaction centers (FvO/FmO), the photochemical quenching (qp) and the actual PSⅡ efficiency ((Φ)PSⅡ) all decreased while the non-photochemical quenching (NPQ) increased greatly with increasing NaCl concentration.Pn,-1FvO/FmO, qp and (Φ)PS had an ameliorative trend in the presence of 100 mmol L NaCl treatment, but this phenomenon did not appear in 200 mmol L-1 NaCl treatment.