ObjectiveTo analyze the impact of resection margin length on postoperative clinical outcomes in patients with Siewert type Ⅱ/Ⅲ adenocarcinoma of the esophagogastric junction (AEG) and to investigate the independent risk factors influencing postoperative positive resection margin. MethodsBased on sample size estimation, 173 patients with AEG admitted to the 980th Hospital of Joint Logistics Support Force of Chinese People’s Liberation Army from July 2022 to January 2025 were prospectively enrolled and divided into 3 groups according to the proximal resection margin length: <20 mm group, 20–30 mm group, and >30 mm group. Baseline data were compared among the 3 groups, and differences in clinical outcomes among patients with different resection margin lengths were analyzed. Multivariate logistic regression analysis was used to identify independent risk factors for positive resection margin, and receiver operating characteristic (ROC) curve was used to evaluate the discriminative ability of these independent factors for postoperative positive resection margin. Stratified analysis by resection margin length intervals was performed to quantify the association between proximal resection margin length and risk of positive margin, and subgroup analyses were conducted to explore the consistency of this association across different clinicopathologic subgroups. ResultsThere were no statistically significant differences in baseline data such as gender, age among the three groups (P>0.05). Among the three groups, patients in the 20–30 mm group had the shortest operative time and time to first postoperative ambulation (P<0.05), the lowest percentage of body weight loss (P<0.05) and the highest hemoglobin and albumin levels (P<0.05) on postoperative month 6. Multivariate logistic regression analysis showed that tumor length ≥5 cm [OR (95%CI)=4.500 (2.519, 8.038), P=0.008], poorly differentiated pathological type [OR (95%CI)=3.803 (2.098, 6.882), P=0.026], and resection margin length <20 mm or >30 mm [OR (95%CI)=3.997 (1.819, 8.793), P=0.037; OR (95%CI)=4.202 (1.906, 9.252), P=0.031, respectively] were independent risk factors for postoperative positive resection margin. The areas under the ROC curve for these three factors individually and their combination in predicting positive resection margin were 0.765, 0.726, 0.702, and 0.847, respectively. The risk-stratified analysis for positive resection margins revealed that, compared with a superior resection margin length of 20–25 mm, the risk of margin positivity significantly increased at lengths of 15–20 mm and 30–35 mm [OR (95%CI)=6.609 (1.816, 24.034), P=0.004; OR (95%CI)=6.618 (1.832, 23.973), P=0.004]. Subgroup analyses showed that the correlation between resection margin length and positive margin was more pronounced in patients with tumor length ≥5 cm and poorly differentiated pathology (Pinteraction<0.05). ConclusionsFor patients with Siewert type Ⅱ/Ⅲ AEG, maintaining the superior resection margin length within the 20–30 mm range during surgical resection can ensure oncological radicality while optimizing postoperative recovery and nutritional status. For patients with tumor length ≥5 cm and poorly differentiated pathology, greater emphasis should be placed on accurate measurement and frozen section confirmation during surgery.