Objective:To explore the characteristics of Siewert classification and microsatellite instability(MSI) and HER2 expression in adenocarcinoma of esophagogastric junction (AEG).Methods:The clinicopathological data of gastric adenocarcinoma from May 2019 to November 2020 were retrospectively analyzed. The patients were divided into two groups: AEG group and non AEG group. The composition ratio of Siewert type of AEG was counted, and the relationship between tumor size and Siewert type was analyzed. The MSI status and HER2 expression status of AEG and non AEG were statistically compared. The measurement data of normal distribution were expressed as mean ± standard deviation( Mean± SD), the comparison between groups were by t test, the comparison of count data between groups were by Chi-square test. Results:A total of 328 consecutive cases of gastric adenocarcinoma were collected, including 242 cases of AEG and 86 cases of non AEG. The Siewert classification of AEG was mainly type Ⅱ (151 cases, 62.40%), followed by type Ⅲ (86 cases, 35.54%) and type Ⅰ (5 cases, 2.07%). The analysis of the relationship between the size of the tumor length and the number of Siewert type showed that the number of Siewert type Ⅱ cases decreased and the number of Siewert type Ⅲ cases increased with the increase of the tumor size. MSI status was detected non selectively in 192 cases of gastric adenocarcinoma (140 cases of AEG and 52 cases of non AEG). There were 12 cases of MSI (6.25%), 2 cases of MSI-H (1.04%) and 10 cases of MSI-L (5.21%). There was no significant difference in MSI ratio between AEG group and non AEG group ( P>0.05). All MSI cases were negative or weakly positive for PMS2. The expression of HER2 was detected by immunohistochemistry in 313 cases of gastric adenocarcinoma, except 15 cases of PTIS/T1a. There were 30 cases (9.58%) with HER2 expression 3+ . Thirty-two cases (10.22%) expressed HER2 (2+ ), of which 7 cases were detected by fluorescence in situ hybridization (FISH), and 3 cases were positive. The proportion of HER2 (3+ ) in AEG was significantly higher than that in non AEG group ( P<0.05). Conclusions:The main type of AEG was Siewert type Ⅱ. AEG may mostly occur between 1 cm above the esophagogastric junction and 2 cm below the esophagogastric junction; For endoscopic screening of early AEG, more attention should be paid to this area of stomach. Siewert type Ⅲ may be derived from the development of Siewert type Ⅱ. The incidence of microsatellite instability in gastric cancer is low. Compared with other gastric adenocarcinoma, AEG has no specificity in MSI. The MSI of AEG was mainly the expression defect of PMS2. Compared with other gastric adenocarcinoma, there are more HER2 overexpression in AEG.