Objective To analyze clinical characteristics affecting survival outcomes in gastric adenocarcinoma(GAC)patients with second primary malignancies(SPM)and construct a predictive model with a web-based calculator.Methods Patients diagnosed with GAC between January 2010 and December 2017 in the SEER database(n=24 085)were analyzed,comparing non-SPM(n=22 963)and SPM cohorts(n=1 122).SPM patients were randomized(3:1)into training(n=842)and internal validation cohorts(n=280).Univariate/multivariate Cox regression identified prognostic factors for model construction.Model performance was evaluated via ROC curves,calibration plots,and decision curve analysis(DCA).A web-based calculator was deployed using DynNom(https://kunma697.shinyapps.io/dynnomapp-1/).External validation used 192 SPM patients diagnosed at Yantai Yuhuangding Hospital(2010-2017).Results χ2 tests revealed SPM patients had higher age(56.3%),earlier T-stage(T1:29.2%;T2:10.5%),predominant gastric cardia involvement(43.7%),fewer distant metastases(12.3%),and higher rates of radiotherapy(32.5%)and surgery(77.2%)vs.non-SPM(P<0.05).Cox analyses identified GAC primary site,T-stage,SEER stage,radiotherapy/surgery history,plus SPM grade/stage/treatment history as significant predictors(P<0.05).AUCs in the training cohort were 0.771(95%CI:0.722～0.820),0.839(95%CI:0.796～0.882),and 0.836(95%CI:0.792～0.879)for 1-/3-/5-year survival;internal validation showed 0.751(95%CI:0.700～0.801),0.746(95%CI:0.695～0.797),and 0.772(95%CI:0.723～0.821);external validation yielded 0.713(95%CI:0.648～0.778),0.805(95%CI:0.749～0.861),and 0.851(95%CI:0.801～0.901).Calibration indicated high prediction-actuality concordance;DCA confirmed clinical utility.Conclusion The model and web calculator incorporating GAC/SPM characteristics effectively predict SPM patient prognosis.