Understanding the behavior of interfacial system plays crucial role in polymer processing, lubrication, adhesion, clay swelling, etc. Over the past several decades, wall slip or instability at interfaces has been studied in experimental and computational methods. Although past works have revealed many properties of these phenomena, there are still some questions about molecular mechanism. In this study, we reported the mechanism and dynamics of confined short-chain polymeric melts using molecular dynamics simulation. We performed non-equilibrium molecular dynamics (NEMD) simulations of linear polyethylene (PE) melts confined between parallel solid walls under steady-shear flow at 450K. Through rigorous molecular analysis, it was found that motion of single chain at interfaces differs from hairpin motion generally shown in bulk under shear flow. This interfacial motion could be categorized as 4 representative motions according to configuration shape and degree of interaction with wall atoms. We also calculated the residence time and slip length of representative configurations at interface. And we present in detail results for various rheological, structural and dynamic properties of our systems. It was recognized that the distributions of each representative configuration with Weissenberg number are important factor in determining interfacial phenomena such as slip or instability. In addition, we probed there was specific Weissenberg number interval of the motions showing both stick and slip motions using chain end-to-end distance or radius of gyration. This will help us understand molecular-level slip and instability (e.g., melt fracture) in polymer processing.