Objective To explore the features of extracellular matrix of pelvic floor muscles in pregnant women with stress urinary incontinence (SUI). Methods Seventy-one primiparas with regular prenatal examination undergoing vaginal delivery were selected, and were divided into SUI group (n=33) and non-SUI group (n=38). Deep pelvic floor tissues were obtained after delivery, HE staining was conducted, electronic microscopy was performed for ultrastructure observation, and quantitative analysis of collagen and elastic fibers was carried out with Masson's trichrome staining and Verhoeff-van Gieson staining. Besides, another 3 women without pregnancy were served as non-pregnant controls. Results There were vacuolar degeneration, filament alteration and edema in smooth muscle cells of deep pelvic floor tissues in all pregnant women. There were plentiful of collagen and elastic fibers around striated muscles in non-SUI group. The main component of deep pelvic floor tissues in SUI group was smooth muscle, and there were sparse collagen and elastic fibers around smooth muscles. The relative contents of collagen fibers and elastic fibers of pelvic floor muscle tissues in SUI group were significantly lower than those in non-SUI group (P<0.05). Conclusion The extracellular matrix structure of pelvic floor muscle tissues in pregnant women with SUI is loose, with less collagen fibers and elastic fibres, which may be associated with the occurence of SUI.