AbstractBackgroundImmune checkpoint inhibitor (ICI) combination therapies have shown promise in the first‐line treatment of advanced biliary tract cancer (BTC). However, the best partner remains to be validated. Moreover, progress on biomarkers predicting the efficacy of ICI in BTC is slow. This study aimed to assess the efficacy and investigate reliable predictive biomarkers of programmed cell death protein‐1 (PD‐1) antibody combination therapy in the first‐line treatment of advanced BTC.MethodsClinical data from patients with advanced BTC who received chemotherapy or anti‐PD‐1 combination therapy as first‐line were collected. The primary outcome was overall survival (OS). Biomarkers, including peripheral blood inflammation scores, genetic alterations, and tumor microenvironment were investigated.FindingsSixty‐four patients were recruited and divided into four treatment groups: chemotherapy, anti‐PD‐1 plus chemotherapy, anti‐PD‐1 plus targeted therapy, and triple group (anti‐PD‐1 plus chemotherapy and targeted therapy). The median OS was 7.9, 11.3, 12.8, and 28.7 months, respectively. Compared to chemotherapy, mOS significantly prolonged in the triple group (p = 0.031). It showed that patients with five different peripheral blood inflammation scores had significantly prolonged mOS (p < 0.05). Genetic testing results suggested that patients with poor survival all had TP53 mutations and higher levels of KRAS and ERBB2 mutations. Low FOXP3/CD8 ratio was associated with prolonged OS (p = 0.029). With CD4‐low, CD8‐high, CD56‐positive, CD163‐high, FOXP3‐high and MPO‐high in TME as one factor, we calculated PLUS score according to the number of factors. The high‐PLUS (>2) group showed significantly superior OS (p = 0.003).InterpretationFirst‐line anti‐PD‐1 combination therapy was superior to chemotherapy, and triple therapy significantly improved survival. Peripheral blood immune‐inflammation score, FOXP3/CD8 ratio, and PLUS have potential as biomarkers for predicting the efficacy of first‐line anti‐PD‐1 therapy in advanced BTC.