To explore the risk factors of peritoneal metastasis in T_4a gastric cancer，and construct a line graph model for the training group，and conduct internal and external verification.

Clinical data of 394 patients with stage T_4a gastric cancer who underwent gastrectomy +D2 lymph node dissection from January 2011 to December 2017 were retrospectively analyzed，including 224 patients from January 2011 to December 2015 as the training set，and 170 patients from January 2016 to December 2017 as the verification set. Based on the preoperative imaging findings to determine presence of peritoneal metastasis and finally confirmed by pathological data，using SPSS 24.0 software by t test，rank and inspection or chi-square statistical analysis of risk factors for peritoneal metastases，by single factor and multiple factors Logistic regression filter T_4a period the potential risk factors of peritoneal metastasis in gastric cancer patients，R software（version 4.0.2）was used to build the line graph model. Bootstrap method was used for internal verification，ROC curve was used to evaluate the model's compliance，95%CI was calculated，and calibration curve was drawn to evaluate the model's compliance. DCA curve was drawn to evaluate the clinical benefit of the model.

Among 224 patients in the training set，37（16.5%）patients developed peritoneal metastases，while 23（13.5%）patients in the verification set developed peritoneal metastases. Multivariate analysis showed that carbohydrate antigen 125（CA125），ascites，preoperative albumin（ALB）and tumor differentiation degree were independent risk factors for peritoneal metastases in gastric cancer patients. ROC curve drawing results indicated that the AUC area under the curve of internal training set was 0.783（95%CI：0.699-0.867），and the AUC area under the curve of external validation set was 0.848（95%CI：0.763-0.932）. And the model of column graph established by this method has good differentiation，calibration and clinical benefit.

The line graph model based on four independent risk factors has a good degree of differentiation and calibration for the occurrence of peritoneal metastasis in patients with stage T_4a gastric cancer，which can be used for preoperative assessment of the risk of peritoneal metastasis in patients with stage T_4a gastric cancer，and has certain clinical promotion and reference value.