Background Recently, the incidence of missed miscarriage has gradually increased, and medical abortion is a common method to terminate a pregnancy. In the process of medical abortion, massive vaginal bleeding takes place, leading to emergency surgical haemostasis. Emergency surgery may produce infection and organ damage. Our study aimed to investigate the high-risk factors for massive haemorrhage during a medical abortion. Methods A total of 1062 missed miscarriage patients who underwent medical abortion participated in this retrospective study.According to the amounts of bleeding, the patients were divided into massive haemorrhage group and control group.By comparing the general conditions of the two groups, such as fertility history, uterine surgery history, uterine fibroids, etc, the high-risk factors for massive haemorrhage during medical abortion were identified. Results Relative to the control group, the massive haemorrhage group exhibited a higher proportion of patients with a previous artificial abortion (51.9% vs. 38.1%, P = 0.001). Additionally, the massive haemorrhage group had a lower percentage of first-time pregnant women (32.1% vs. 40.4%) and a higher proportion of women with shorter pregnancy intervals (44.9% vs. 33.1%, P = 0.03). Furthermore, there were notable differences between the two groups regarding maximum fibroid size, the duration of amenorrhea, and gestational week (P < 0.05). Conclusion In this study, we determined that a history of artificial abortion and an amenorrhea duration > 11 weeks represented high-risk factors for massive vaginal bleeding during medical abortion in missed miscarriage patients.