Pot experiments were carried out to study the effects of NaCl stress on the characteristics of photosynthesis and chlorophyll fluorescence in Portulaca oleracea L. Eight different concentrations of NaCl were applied: 0, 25, 50, 75, 100, 150, 200, 250 mmol?L-1. The results showed as follows : The plant fresh weights and heights of seedlings had no difference with the control when the concentration of NaCl was 25mmol?L-1, but decreased with increasing NaCl concentration when the NaCl concentration was above 25 mmol?L-1. When stressed by NaCl, inhibition was the first biomass, then followed by plant height. With increasing NaCl concentration, net photosynthetic rate (Pn) decreased, intracellular CO2(Ci) increased, and the magnitude of changes of Pn and Ci compared with the control increased with increasing NaCl concentration. Minimal fluorescence (Fo) , maximum fluorescence (Fm) , variable fluorescence (Fv) , stable fluorescence (Fs) , difference between Fs and Fo (△Fo) , PSⅡ potential fluorescence efficiency (Fv/Fo), PSⅡmaximum fluorescence efficiency (Fv/Fm) and photochemical quench (qP) of seedlings all decreased when stressed by NaCl, while NPQ increased. And at low concentrations NaCl (0-50 mmol?L-1) stress, the fluorescence parameters decreased less than other high concentrations of NaCl stress. So Portulaca oleracea L. was injured by photo-inhibition on NaCl stress, and at low concentrations, plant applied more light energy to photochemical reaction to maintain a high net photosynthetic rate, the degree of photo-inhibition was lower, therefor, NaCl stress had little effect on growth.