Direct chemical modification of native antibodies in a site-specific manner remains a great challenge due to the complicated structural characteristics of antibodies. Ligand-directed conjugation can achieve the selective modification of antibodies, but usually requires multiple extra steps for ligand release and cargo assembly. Herein, we report a novel, traceless strategy to enable the facile and efficient one-step synthesis of site-specific antibody-drug conjugates (ADCs) by harnessing a thioester-based acyl transfer reagent. The designed reagent, consisting of an optimized Fc-targeting ligand, a thioester bridge and a toxin payload, directly assembles the toxin precisely onto the K251 position of native IgGs and simultaneously self-releases the affinity ligand in one step. With this traceless, one-step method, we successfully synthesized a series of K251-linked ADCs from native Trastuzumab. These ADCs demonstrated excellent homogeneity, thermal stability, and both in vitro and in vivo anti-tumor activity. This strategy is equally efficient for IgG1, IgG2, and IgG4 subtypes. The current work presents a robust, traceless, site-specific antibody functionalization via a self-releasing strategy, and offers a novel acyl transfer design for broad potential application in ligand-directed protein modifications.