Objective: To investigate the factors affecting postpartum stress urinary incontinence (PSUI) among lying-in women at ages of 35 years and older, so as to provide insights into PSUI prevention. Methods: Lying-in women at ages of 35 years and older receiving postpartum examinations were sampled using a convenient sampling method from Hangzhou Obstetrics and Gynecology Hospital during the period from January 2021 to April 2022. Participants' demographic data, type of delivery, birth weight, diastasis recti abdominis, pelvic floor muscle strength injury, pelvic floor muscle training were collected, and the development of PSUI was evaluated using the International Consultation on Incontinence modular questionnaire. The factors affecting the development of PSUI were identified among lying-in women at ages of 35 years and older using a multivariable logistic regression model. Results: A total of 230 questionnaires were allocated, and 226 valid questionnaires were recovered, with an effective recovery rate of 98.26%. The lying-in women had a mean age of (37.30±2.11) years, and 75.66% had a pre-pregnancy body mass index (BMI) of 18.5 to 24.0 kg/m2. There were 29 women with postpartum BMI of 24.0 kg/m2 and greater (12.83%), 201 women with gestational weeks of 37 weeks and greater at delivery (88.94%), 105 women with vaginal delivery (46.46%), 20 women with neonatal birth weights of 4 000 g and higher (8.85%), 149 women with diastasis recti abdominis (65.93%), 154 women with pelvic floor muscle strength injury (68.14%). The prevalence of PSUI was 25.22% among the study subjects. Multivariable logistic regression analysis showed that vaginal delivery (OR=4.061, 95%CI: 2.124-7.763), postpartum BMI of 24 kg/m2 and higher (OR=1.903, 95%CI: 1.275-3.288), neonatal birth weight of 4 000 g and higher (OR=2.108, 95%CI: 1.420-4.135), diastasis recti abdominis (OR=1.487, 95%CI: 1.110-2.169) and pelvic floor muscle strength injury (OR=2.924, 95%CI: 1.726-4.803) were risk factors for PSUI among lying-in women at ages of 35 years and older, and pelvic floor muscle training was a protective factor for PSUI among lying-in women at ages of 35 years and older (OR=0.410, 95%CI: 0.216-0.780). Conclusions The development of PSUI correlates with the type of delivery, postpartum BMI, neonatal birth weight, diastasis recti abdominis and pelvic floor muscle strength injury among lying-in women. Reasonable weight control and active pelvic floor muscle training may facilitate the prevention of PSUI.