Abstract: In order to obtain better knowledge about the physiological mechanism of endurance shade in different soybean cultivars, two soybean cultivars soybean with different levels of shade endurance was plant under two different light condition and the characteristics of expansins in soybean internodes and their responses to shade stress were studied. Expansin proteins were extracted from the cell walls of soybean internodes by the methods of CaCl2 and Hepes. The activities of expansins were measured with an extensometer. The expansins extracted with CaCl2 had higher activity than Hepes. The results demonstrated for the first time the presence of expansins in soybean internodes, it could induce extension of cell walls in soybean and in cucumber hypocotyls. The expansins activities of endogenous and reconstituted cell wall extension depended on acidic pH. The changes in activity of expansins in different soybean cultivars in response to shade stress suggest that expansins may play a significant role in the endurance of relay cropping soybean to shade stress.