Objective Previous studies on the surgical treatment of cesarean scar pregnancy （CSP） have primarily focused on traditional dilation and curettage. In recent years，hysteroscopic resection of the scar site pregnancy lesion has gradually become an important method for CSP treatment，but it still faces the risk of retained products of conception （RPOC）. This study aims to explore the risk factors associated with RPOC after hysteroscopic surgery for CSP by integrating multi-dimensional objective indicators. Methods A retrospective analysis was conducted in 1022 patients diagnosed with CSP and treated with hysteroscopic surgery at the Obstetrics and Gynecology Department of Peking University Third Hospital from January 2014 to October 2024. Patients with postoperative RPOC were included as the study group，and those without RPOC as the control group. Differences in medical history，clinical manifestations，laboratory and imaging examinations，and other aspects were compared between the two groups. Results The overall incidence of RPOC after hysteroscopic surgery for CSP was 4.0% （41/1022）. Univariate analysis indicated that patient’s age，uterine position，gravidity and parity，number of cesarean sections，history of CSP，history of intrauterine surgery，gestational age at this CSP diagnosis，presence of fetal heart activity，and preoperative clinical symptoms （abdominal pain/ vaginal bleeding） were not directly related to postoperative RPOC（all P>0.05）. After incorporating interaction analysis，multivariate logistic regression analysis revealed that a mean gestational sac diameter≥15 mm and a residual myometrial thickness <2.5 mm at the lower uterine segment scar site measured by magnetic resonance imaging （OR=4.262，95% CI 2.179-8.337，P<0.001） were independent risk factors for RPOC. Conclusions RPOC after hysteroscopic surgery for CSP is closely related to the diameter of the gestational sac and the residual myometrial thickness at the lower uterine segment scar site. A comprehensive assessment and strict indication control should be conducted before CSP patients undergo hysteroscopic surgery. Early detection and early intervention are important ways to reduce the incidence of RPOC.