Corrosion of carbon steel (CS) during the pickling process is a tough issue, and adding corrosion inhibitors is an effective method to solve this problem. In this work, a novel pyrazine derivative 5,5′-(pyrazine-2,5-diyl)bis(4H-1,2,4-triazole-3,4-diamine) (PDTD) was synthesized from pyrazine-2,5-dicarboxylic acid (PDA). PDA and PDTD were utilized as corrosion inhibitors for CS in HCl medium, and their inhibition performance was evaluated using electrochemical tests, surface analysis, and first-principles calculations. Electrochemical measurements reveal that the structural modification of PDA effectively upgrades inhibition capability, increasing the optimal efficiency from 95.7% (PDA) to 98.3% (PDTD). Unlike traditional corrosion inhibitor theory research, first-principles calculations based on DFT combined with partial density of states (PDOS) and chemical bonding configuration analysis reveal for the first time the interfacial inhibition mechanism of PDTD on carbon steel surfaces. First-principles calculations confirm the formation of Fe–N and Fe–C bonds at the inhibitor–steel interface, providing clear evidence of chemisorption.