Objective To evaluate postpartum pelvic floor dysfunction (PFD) in pregnant women with prenatal pelvic ring disease after vaginal natural delivery by 2D, 3D and 4D ultrasound. Methods The primiparas undergoing regular prenatal examinations and vaginal natural delivery in the hospital were enrolled as the research objects between January 2020 and March 2023, including 150 cases with pelvic ring disease during pregnancy in case group. According to the principle of propensity matching, 150 pregnant women without pelvic ring disease were enrolled as control group. All underwent ultrasound examination before delivery to detect pubic symphysis space and sacroiliac joints. The pelvic floor function was assessed by 2D, 3D and 4D ultrasound at 42d after vaginal delivery. The occurrence of postpartum PFD was compared between the two groups. Results BND parameters by 2D ultrasound in case group were lower than those in control group (P<0.05), while BSD parameters, PUA parameters in rest and Valsalva state were higher than those in control group (P<0.05). LHLR and LHAP parameters in rest and Valsalva state by 3D ultrasound in study group were higher than those in control group (P<0.05). The maximum area of levator hiatus in rest and Valsalva state by 4D ultrasound in study group was larger than that in control group (P<0.05), and thickness of levator ani was thinner than that in control group (P<0.05). The incidence of PFD (uterine prolapse, encephalocele of anterior vaginal wall, encephalocele of posterior vaginal wall, stress incontinence) in case group was higher than that in control group (P<0.05). Conclusion Compared with women without pelvic ring disease before delivery, relevant parameters are worse and incidence of postpartum PFD is higher in those with pelvic ring disease after after vaginal natural delivery by 2D, 3D and 4D ultrasound.