Abstract: The changes in photosynthetic properties of filamentous cyanobacterium Anabaena sp.PCC 7120(A. 7120) in response to NaCl stress had been studied.It showed that both the net photosynthetic oxygen evolution rates and respiration rates were decreased more quickly with increasing NaCl concentrations when NaCl concentrations exceed 0.4 mol/L were. The absorption spectra had been shown that the photosynthetic pigments of A. 7120 under NaCl stress were unchanged,but the total contents of phycobiliproteins decreased and those of carotenoids increased.The low temperature fluoresence emission spectra suggested that the energy transfer from phycobiliprotein to photosystem I in A.7120 under NaCl stress was depressed.And the chlorophyll a fluoresence induction kinetics analysis displayed that changes in Fv/Fm ratios of A.7120 was consistent with those of the net photosynthetic oxygen evolution rates in response to NaCl stress.