The posterior fornix generally refers to the posterior vaginal fornix. The female vagina is located in the center of the lower part of the true pelvis, a tube that is wide at the top and narrow at the bottom, and the circumferential crypt between the cervix and the vagina is called the vaginal fornix. The vaginal fornix is divided into four parts according to its position: the anterior fornix, the posterior fornix, the left fornix, and the right fornix, and the posterior fornix is located behind the vaginal fornix. It is at the lowest part of the pelvic cavity. Clinically, ectopic pregnancy ruptures and bleeds, or there are other causes of intra-abdominal bleeding, and blood collects in the lowest part of the pelvic cavity. At this time, the posterior vaginal fornix can be punctured through the posterior vaginal fornix, and if non-coagulated blood is drawn from the puncture, it can assist in the diagnosis and can be used as a basis for surgery.