This is a phase III, multi-center study to evaluate the efficacy and safety of chiauranib plus plus toripalimab, albumin-paclitaxel and gemcitabine as first-line therapy in patients with metastatic pancreatic ductal adenocarcinoma. The study includes two period: Run-in period and Randomized controlled period. The Run-in period is a single-arm, open-label study enrolling approximately 20 participants, who received the combination therapy of chiauranib plus toripalimab, albumin-paclitaxel and gemcitabine. The Randomized controlled period is a randomized, double-blind, parallel-controlled study enrolling approximately 538 participants, who are 1:1 randomly assigned to the experimental arm(chiauranib plus Toripalimab, albumin-paclitaxel and gemcitabine) or the control arm (Chiauranib placebo plus toripalimab placebo, albumin-paclitaxel and gemcitabine).