The　influence　of　filler　wire　configuration,　such　as　size　and　geometry,　on　the　deposition　rate　(DR)　and　bead　formation,　has　been　studied　in　wire　arc-based　directed　energy　deposition　(WADED),　but　the　fundamental　physics　underlying　its　effect　on　wire　melting　and　melt　pool　dynamics　remains　unclear.　In　this　paper,　a　series　of　plasma　arc-based　DED　(plasma-DED)　experiments　were　conducted　to　investigate　the　impact　of　five　different　filler　wire　configurations　on　DR　and　bead　dimensions.　The　coupling　behaviours　of　wire　melting,　metal　transfer　and　melt　pool　dynamics　under　the　five　filler　wire　configurations　were　also　simulated　numerically　using　the　authors＇　recently　developed　wire-feeding　model.　The　calculated　wire　melting　and　bead　cross-sections　are　consistent　with　the　experimental　images　and　measurements.　The　results　demonstrate　that　the　filler　wire　significantly　affects　the　highest　DR　by　altering　wire　melting　and　metal　transfer　behaviours　through　changes　in　arc　energy　absorption.　The　filler　wire　with　a　rhombus　geometry　which　is　closer　to　a　Gaussian-like　arc　distribution　than　the　flat　wire　was　shown　to　get　higher　DR　and　more　stable　metal　transfer.　Furthermore,　different　filler　wire　configurations　lead　to　distinct　melt　pool　behaviours,　including　temperature　distribution　and　flow　velocity,　due　to　various　metal　transfer　behaviours　and　arc　shading　effects.　This　study　sheds　light　on　the　fundamental　physics　underlying　the　impact　of　filler　wire　on　wire　melting　and　bead　formation　for　the　first　time.　The　methods　and　findings　can　guide　improving　DR　and　controlling　bead　shape　in　the　plasma-DED　process.