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Bacterial pneumonia poses a global health threat, worsened by drug-resistant bacteria, underscoring the urgent need for new antimicrobial agents. Antimicrobial peptides show promising broad-spectrum activity and low resistance development. This study reports a linear antimicrobial peptide 1003 consisting of four repeating units of (D-tryptophan)-(D-arginine)-(D-lysine). It exhibits high stability and strong antimicrobial activity against multidrug-resistant bacteria like methicillin-resistant Staphylococcus aureus and Klebsiella pneumoniae, with improved biocompatibility.