Objective:To evaluate the clinical value of four dimensional ultrasound combined with pelvic floor surface electromyography in the assessment of bladder prolapse in primipara with different delivery modes.Methods:A total of 413 primipara 6-8 weeks after full-term delivery were selected from the obstetrics clinic of the Second Affiliated Hospital of Hainan Medical College from October 2021 to September 2023. They were divided into natural delivery group(349 cases) and cesarean section group(64 cases ). The characteristics of 4D pelvic floor ultrasound in the two groups were analyzed and summarized. Then 64 cases of primipara with pelvic floor surface electromyography were divided into bladder prolapse group (46 cases) and no bladder prolapse group(18 cases). The characteristics of four dimension ultrasound combined with pelvic floor surface electromyography in the two groups were analyzed, and the related risk factors of bladder prolapse were analyzed.Results:The bladder neck mobility and urethral rotation angle in the natural delivery group were higher than those in the cesarean section group (all P<0.05). Compared with the cesarean section group, the incidence of obvious prolapse and urethral infundibulation was higher in the natural delivery group, while the incidence of mild prolapse was lower than that in the cesarean section group (all P<0.05). Bladder neck mobility, urethral rotation angle and levator ani hiatal area in the prolapsed bladder group were higher than those in the non-prolapsed bladder group (all P<0.05). The maximum value of fast muscle stage and the mean value of slow muscle stage in the group with prolapse were lower than those in the group without prolapse (both P<0.05). Univariate Logistic regression analysis found that the risk factors for bladder prolapse were increased birth weight, natural delivery, increased bladder neck mobility, posterior bladder angle was opened, increased urethral rotation angle, increased levator ani hiatal area, decreased maximum value of fast muscle stage and decreased mean value of slow muscle stage. Multivariate Logistic regression analysis showed that increased ani hiatal area ( OR=2.216, P=0.015) and decreased maximum value of tater muscle stage ( OR=0.847, P=0.035) were risk factors for bladder prolapse. Conclusions:Pelvic floor ultrasound combine with pelvic floor surface electromyography can qualitatively and quantitatively evaluate the changes of pelvic floor muscle structure and function in postpartum women, and diagnose bladder prolapse and its degree. The increase of levator ani hiatal area and the decrease of maximum value of tater stage may be the risk factors for bladder prolapse at 6-8 weeks postpartum in primiparas.