The common vetch (Vicia sativa) is an important annual legume forage at high altitudes. However, the phenomenon of pod shattering when it matured would casue serious loss of seeds. With V. sativa cultivar Lanjian NO.3 as the research object, this study analyzed dynamic changes in pod morphological, moisture content, surface structure of ventral suture, anatomical structure of ventral suture cross section during pod development in order to explore the pod shattering mechanism of V. sativa and provide theoretical basis for determining its optimal seed harvesting time in production. The results showed that: (1) When the pod of Lanjian NO.3 turned to light brown at about 25-30 days after peak anthesis, it had completed physiological maturity and its size and dry weight had reached the highest with the moisture content had reduced to the lowest. A crack appeared in the pod ventral suture at 25 days after peak anthesis and it separated completely at 35 days after peak anthesis. (2) The abscission layer cells began to disintegrate at 20 days after peak anthesis. At 25 days after peak anthesis, all the parenchyma cells of endocarp, mesocarp and exocarp began to lose water and shrink wherein the portion of endocarp parenchyma cells began to break. The abscission layer cells and the following parenchyma cells disintegrated completely. The inside cell walls of external valve margin cells broke but the outside cell walls which abnormal thickened still remained intact and connected two fruit valves which made the pod do not crack. During the period from 25 to 30 days after peak anthesis, the parenchyma cells of endocarp, mesocarp and exocarp lost water completely and their cell walls shrank together. At the same time the outside cell walls of external valve margin cells broke into two parts and the two fruit valves of the pod cracked. The research shows that the optimal harvest time of Lanjian NO.3 seeds was the period from 25 to 30 days after peak anthesis when the pod turned to light brown. The abscission layer cells and machine tensile which generated by cell water loss were the main causes of V. sativa pod crack. Conjecture that the thickening and fused cell walls of external valve margin cells were the key structure resistant to pod shattering probably.