A 37-year-old pregnant woman, para 3, was transferred to our hospital at 30 weeks of gestational age with severe acute abdomen. Ultrasonography of the abdomen confirmed that body fluid around the uterus was retained. Paracentesis was performed and she was diagnosed with hemoperitoneum. Exploratory laparotomy was performed and arterial bleeding from the posterior of left round ligament was found. Spontaneous rupture of uterine vessel in pregnancy is associated with a high rate of mortality. Unfortunately, clinical symptoms of hemoperitoneum are non-specific. For the well-being of the mother and fetus, rapid diagnosis and surgical treatment of hemoperitoneum during pregnancy is needed. Non-invasive and easy-to-implement ultrasonography can be performed periodically to help diagnose quickly.