Pancreatic stellate cells (PSCs) constitute the fibrotic tumor microenvironment composed of the stroma matrix, which blocks the penetration of gemcitabine (GEM) in pancreatic adenocarcinoma (PDAC) and results in chemoresistance. We analyzed the expression of α-SMA, collagen type I, and fibronectin by immunohistochemistry of pancreatic cancer tissues and demonstrated that the abundant interstitial stroma is associated with dismal survival. Two desmoplastic pancreatic tumor models are treated with arsenic trioxide (ATO) and GEM to confirm the sensitizing effect of ATO on GEM. RNA-seq was performed to analyze the potential fibrotic genes regulated by ATO. Western blotting, CCK-8 methods, colony formation, and wound healing and transwell assays were utilized to verify that ATO attenuates the tumor-promoting ability of PSCs by inhibiting its activation and decreasing matrix secretion via the PI3K/AKT/AP4/galectin-1 pathway. Furthermore, we developed targeted ATO-loaded nanoparticles self-assembled by poly (D,L-lactide) and poly(ethylene glycol) (PEG-PDLLA) and modified by the single-chain antibody of FAP-α (scAbFAP-α) (scAb-ATO-NPs) to promote the delivery efficiency of ATO to PSCs and enhance anti-tumor effects with gemcitabine. Herein, we elucidate the mechanism of how ATO inhibits the activation of PSCs and enhances the therapeutic effect of GEM. We propose a novel cocktail therapy including scAb-ATO-NPs and GEM, indicating a new perspective in the treatment of PDAC.