It was normally been considered that calmodulin is an entracellular calcium receptor protein, and play important roles in signal transduction pathway of eukaryocyte cells. However, nearly 15 years' works in our lab have proved that plant calmodulin not only exits extracellularly but also has important extracellular functions. Especially in recent years, we have got great process in studying the new functions and the signal transduction pathway of plant extracellular calmodulin. With the research going on in depth, We have to face the problem: how does extracellular calmodulin be regulated when it was in the high concentration calcium circulation of plant cell wall? Here, by using the methods of equilibrium dialyze and activation of NAD kinase, we have studied the influence of different pH on the binding of calcium to calmodulin and the activation effects of calmodulin to its target enzyme. It was founded that besides decreasing the bind- ing affinity of calcium to calmodulin, H⁺could also inhibit calmodulin active NAD kinase. When pH decrease 2 units,it could decrease the sensation of calmodulin to calcium concentration to 10 folds when it active NAD kinase. We suggest that this phenomenon might play some role on the regulation mechanism of extracellular calmodulin. Other possible regulation mechanisms which were distinguishable to intracellular calmodulin were also discussed.