Objective To discuss the value of ultrasonography in diagnosing uterine scar defect during the period of pregnancy after cesarean operation so as to take necessary treatment timely to ensure the maternal and fetal safety. Methods Ultrasonography was used to check the pregnant women in their second or third trimester of re-pregnancy after cesarean operation, and 82 women were diagnosed as uterine scar defect. Among them 49 cases had thin myometrium (< 1.0 mm), and 33 cases had local myometrium deletion. Cesarean was operated again within 12 hours after ultrasound examination. The uterine scars were successfully explored in 77 cases during operation, and the results of ultrasonography before operation and the findings during operatin were compared. Results The diagnostic coincidence rate of ultrasound approach was 87.0% for uterine scar defect and 84.4% for rupure of uterine scar. Conclusions Ultrasonography can accurately assess ulterine scar defect during re-pregnancy after cesarean operation.