Poly　(heptazine　imide)　(PHI),　a　classic　2D　polymeric　photocatalyst,　represents　a　promising　organic　semiconductor　for　photocatalytic　overall　water　splitting　(POWS).　However,　since　the　key　bottleneck　in　POWS　of　PHI　remains　unclear,　its　quantum　efficiency　of　POWS　is　extremely　restrained.　To　identify　the　key　obstacle　in　POWS　on　the　PHI,　a　series　of　PHI　with　different　stacking　modes　is　synthesized　by　tuning　interlayer　cations.　The　structural　characterizations　revealed　that　tuning　the　interlayer　cations　of　PHI　can　induce　rearrangements　in　interlayer　stacking　modes.　Additionally,　charge　carriers　dynamics　uncover　that　optimizing　the　interlayer　stacking　modes　of　PHI　can　promote　exciton　diffusion　and　prolong　the　photoexcited　electron　lifetimes,　thus　improving　the　concentration　of　surface-reaching　charge.　More　importantly,　this　confirms　that　the　POWS　activity　of　PHI　is　closely　correlated　with　the　interlayer　stacking　modes.　This　work　offers　new　insight　into　structural　regulation　for　governing　charge-transport　dynamics　and　the　activity　of　2D　polymeric　photocatalysts.　©　2024　Wiley-VCH　GmbH.