Abstract: Means of the fluorescence induction kinetics were used to distinguish the osmotic factor and the ionic factor during NaCl stress. NaCl, KCl, NaNO3, KNO3, PEG solution of the same osmotic potential were used to treat the winter wheat cultured in Hogland solution for two weeks. The results showed that compared with control, the leaves from the NaCl treatment showed apparent decline in the size of acceptor side electron pool of PSII, PSII activity, the efficiency of primary conversion of light energy, quantum yield and qP; at the same time, NaCl treatment could give rise to great increase in the content of the QB_nonreducing reaction center, while the PEG treatment did not cause such changes. This experiment suggested that the osmotic factor might not be a major cause of NaCl stress to harm photosynthesis. In our experiment it was also shown that KNO3 treatment did not cause obvious damage on photosynthetic properties while NaCl and NaNO3 reatments cause great decline in Fv/Fm, Fv/Fo, the size of acceptor side electron pool and obvious increase in the content of Q-nonreducing reaction center. However, PEG and KCl treatment did not caused such changes. These results suggested that Na+ might be the major toxic ion to harm photosynthesis under salinity stress.