In this work, a novel thiadiazole derivative, 1-phenyl-3-(5-thioxo-4,5-dihydro-1,3,4-thiadiazol-2-yl) thiourea (PTT) was synthesized with 5-amino-1,3,4-thiadiazole-2(3 H)-thione (ATT) and applied as the corrosion inhibitor for N80 carbon steel in dynamic supercritical CO₂ aqueous environment. The inhibition performance and mechanism of PTT were investigated by mass loss and electrochemical measurements, surface characterization, and theoretical calculations. It is found that PTT presents the outstanding inhibitive performance with an inhibition efficiency up to 99.58%, which is significantly higher than those reported corrosion inhibitors. The density functional theory (DFT) calculations reveal that the adsorption of PTT on steel surface is implemented through the bonding of the three S atoms of (thione-thiadiazolely)thiourea fragment, as well as the benzene ring. Compared to ATT, the significant improvement in the inhibition performance of PTT is attributed to the stronger adsorption of PTT by forming more S-Fe bonds and the extra C-Fe bonds.