Objective To investigate the structure and mechanical properties of pods after dehydration and the biomechanical mechanism of spreading pod seed injection due to torsion crack. Methods The layered pods, the cell size and direction at different cellular layers were analyzed by histology, microstructure observation, mechanical property test and high-speed photography. The process of pod ejection was observed, and the principle of pod ejection was summarized. Results The ejection of pods started from the crack of the bottom, and cracked gradually from the bottom to the top. The cell arrangement of two parts of the same pod was opposite. Each pod was divided into 4 layers wherein the first exterior layer and the middle layer were orthogonal to each other. There was a layer of cells between the first exterior layer and the middle layer, of which the cell wall was broken. In the process of dehydration, fibers in the outer layer shrank and fibers in the middle layer stretched. Conclusions Pod fiber will be contracted in the orthogonal direction after dehydration to accumulate elastic performance and generate pre-stress, and finally the pod is cracked to release the pre-stress.