Abstract: Nostoc flagelliforme (Berk. Curtis) Born. et Flah. is a terrestrial filamentous nitrogen-fixing cyanobacterium that is distributed in Algeria, China, France, Czech, Slovakia, Mexico, Mongolia, Morocco, Russia, Somalia and USA. It is distributed on northern and northwestern parts of China and has been used as a health food and herbal medicine by Chinese and other Asian populations over centuries. Mass culture of N. fiagelliforme in an engineered system is sought but has not been available, due to the lack of understanding the resistance mechanism and growth physiology of the organism under extreme variable environments. Arid and semiarid steppe is the habitat of this alga in China and alkaline soil is the main growth substrate. It is interesting that the alga can survive and grow in such atrocious soil. This paper was conducted to study the effect of salt stress on this terrestrial alga. The modified BG11 0 culture medium containing different NaCl concentration: 0, 0.1, 0.2, 0.4, 0.8 and 1.0 mol/L, were used to culture algal mats. Algal mats were kept in BG11 8-11h before experiment for physiological recovery. The algal physiological activity and some special substance, such as MDA, polysaccharides, carotenoid and proline, were tested after 12h treatment under 25±0.5℃,40μmol/m 2/s. The results indicated that NaCl stress reduced net photosynthesis, respiration and PSⅡ activity. When NaCl in medium is beyond 0.2mol/L, there were marked decrease in net photosynthesis, respiration and photochemical efficiency. Due to salt stress, permeability of plasma membrane, MDA contents (the production of lipid peroxidation), carotenoid content and polysaccharides content increased, free water content (FWC) and FWC/BWC (bound water content) decreased, while proline content did not change, it meant there is no significant effect of proline in this alga to response to high salt stress. It could be concluded that algal physiological activity was inhibited under salt stress, high salt concentration can lead to increase in the lipid peroxidation and ralative permeability, alter the pigments and influence algal physiological function. On the other hand, this alga also had a certain extent resistance ability to salt stress. Content of carotenoid and polysaccharides were increased to eliminate free radical and regulate osmotic pressure, respectively