Abstract: In this study, the effects of exogenous Ca2+ on the membrane permeability and photosynthetic characteristics of Anabaena sp.PCC7120 cells under simulated microgravity were studied. Simulated microgravity was provided by a slowly horizontal rotating clinostat. Physiological characteristics of Anabaena sp.PCC7120 cells in three different content of Ca2+ were measured. During 72h of treatment, the membrane permeability of Anabaena sp.PCC7120 cells was higher 16% in Ca2+-free medium than that in the control (0.245mmol/L), and higher 45% than that in medium of high concentration of Ca2+ (0.490mmol/L). The photochemistry efficiency (expressed as Fv/Fm) of Anabaena sp.PCC7120 cells decreased under simulated microgravity, but the range of decreasing was different in three different treatments. In Ca2+-free medium, the value of Fv/Fm quickly decreased at the first day, and the decreasing range reached 35% at the third day. However, in the Ca2+-complete medium, the value of Fv/Fm decreased slowly. With increasing concentration of Ca2+ in medium, the decreasing range was only 26% in medium of high concentration of Ca2+(0.490mmol/L) at the end of experiments. The treatment with Ca2+ channel inhibitor showed that the second messenger was involved in the protective process. Experimental results showed that increased Ca2+ in medium could alleviate membrane permeability induced by simulated microgravity, and lessened the decreasing range of photochemistry efficiency in Anabaena sp.PCC7120 cells. Exogenous Ca2+ might enhance stabilization of plasma membrane. Moreover, exogenous Ca2+ might alleviate the microgravity-induced damage of photosynthesis of Anabaena sp.PCC7120 cells.