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How to improve the bonding strength of polymers to metals has been one of the challenges in joining fields. It is generally assumed that laser transmission joining is better than laser conduction joining (LCJ) for transparent polymers, and few studies have been focused on LCJ. However, by introducing beam oscillation, an excellent result was obtained in the LCJ of transparent polyethylene terephthalate (PET) to 304 stainless steel. The interface defects of thermal decomposition and bubbles could be eliminated or reduced more efficiently in oscillating laser conduction joining (O-LCJ) rather than transmission joining. Correspondingly, the tensile shear force of joint O-LCJ could be increased by 23.8%, and the plasticity characterized by tensile displacement could be increased by seven times. The improvement mechanism was attributed to two factors by calculating the interface energy distribution and analyzing the force state at the interface. One is the homogenization of interface energy distribution caused by beam oscillation, which decreases the degradation and destruction of polymer macromolecular chains induced by high temperature. The other is the formation of interface bi-directional forces that both inhibit the porosity formation and intensify the chemical reactions. The results bring new insights and provide a new pathway to improve the joining performances of dissimilar polymers to metals.