In　this　letter,　we　investigate　iterative　decoding　for　convolutional-coded　physical-layer　network　coding　operated　with　bit-interleaved　coded　modulation　(BICM-PNC)　over　two-way　relay　channel　(TWRC).　The　superimposed　constellation　at　the　relay　from　two　users　may　not　remain　the　same　characteristic　as　the　original　modulation　at　the　users.　Thus,　we　first　propose　a　design　of　mapping　for　PAM　modulations　at　the　users　and　enable　the　superimposed　constellation　of　the　maximum　squared　Euclidean　weight　(MSEW).　Then,　we　analyze　the　PNC　mappings　by　means　of　an　extrinsic　information　transfer　(EXIT)　chart.　The　analysis　and　simulation　results　show　that　the　proposed　mapping　can　provide　a　good　match　to　a　given　outer　channel　code.　In　particular,　it　achieves　much　better　error　performance　than　the　conventional　semi-Gray　and　binary-bit　mappings　at　high　SNR　region　for　both　8-PAM　and　16-PAM　modulations.