Objective: To investigate the value of decreased carbohydrate antigen 19-9(CA19-9) kinetics in predicting short-term outcomes and determining prognosis among advanced biliary or pancreatic cancer patients receiving first-or second-line therapy in the real world. Methods : Eighty-nine patients were retrospectively collected with advanced biliary or pancreatic cancer, especially on the CA19-9 dynamics and decline rates at different time points. This study evaluated the association of CA19-9 changes with clinicopathological features, short-term response to antitumor therapy, and survival outcomes. Results : The enrolled patients recorded baseline CA19-9 levels ranging from 1.20 to 65 706.40 U/ml, with a median of 303.11 U/ml. There was no statistical correlation between baseline CA19-9 levels and gender, age, body mass index, primary tumor site, hepatic metastases, pulmonary metastases, lymph node metastases, peritoneal metastases, performance status, treatment lines, and combinations of drug types. Baseline CA19-9 levels were not associated with systemic immunoinflammatory index, prognostic nutritional index, and total bilirubin. A 25% or 50% decrease in CA19-9 after 2-3 therapy courses indicated short-term efficacy in reaching tumor objective remission or disease control. Both combinations of multiple drug types and a 25% decline in CA19-9 after one course of treatment were independent prognostic factors that affected the longer progression-free survival of patients receiving first or second line of treatment. Conclusion Decreased CA19-9 kinetics has specific values in predicting the efficacy and prognosis of advanced biliary or pancreatic cancer.