In　this　paper,　we　investigate　the　performance　of　physical-layer　network　coding　(PNC)　with　hierarchical　modulation　(HM)　over　a　two-way　relay　channel　(TWRC).　While　Gray　mapping　is　the　optimal　for　bit-interleaved　coded　modulation　(BICM)　systems　in　point-to-point　communications,　the　superimposed　constellation　at　the　relay　in　PNC　transmission　does　not　maintain　the　same　characteristics　as　the　original　Gray　mapping　at　the　users.　Thus,　to　address　this　issue　and　obtain　the　optimal　mapping　for　PNC,　we　propose　a　Pseudo-Gray　(Pe-Gray)　constellation　for　user-end　hierarchical　quadrature　amplitude　modulation　(H-QAM).　In　particular,　we　develop　the　design　criterion　for　the　user　constellation　to　avoid　demodulation　ambiguity　and　more　importantly,　to　ensure　that　the　superimposed　constellation　can　be　Gray-mapped.　The　achievable　rate　analysis　and　simulation　results　show　that　the　proposed　Pe-Gray　can　achieve　significant　performance　gains　of　up　to　2　dB　over　the　conventional　Gray,　M3,　and　MSED　constellations　in　power-imbalanced　HM-BICM-PNC.　The　performance　superiority　of　the　Pe-Gray　is　further　demonstrated　over　the　power-imbalanced　channels,　which　also　suggests　its　enhanced　robustness　in　wireless　fading　channels.　IEEE