Unraveling　how　reactive　facets　promote　photocatalysis　at　the　molecular　level　remains　a　grand　challenge,　while　identification　of　the　reactive　facets　can　provide　guidelines　for　designing　highly　efficient　photocatalysts　and　unravelling　the　microscopic　mechanisms　behind　them.　Recently,　a　series　of　polytriazine　imides　(PTIs)　was　reported　with　highly　crystalline　structures;　all　had　a　relatively　low　photocatalytic　activity　for　overall　water　splitting.　Here,　high-angle　annular　dark-field　scanning　transmission　electron　microscopy,　energy　dispersive　spectroscopy　mapping,　and　aberration-corrected　integrated　differential　phase　contrast　imaging　were　used　to　study　PTI/Li+Cl-　single　crystals　before　and　after　in　situ　photodeposition　of　co-catalysts,　showing　that　the　prismatic　{10　(1)　over　bar0}　planes　are　more　photocatalytically　reactive　than　the　basal　{0001}　planes.　Theoretical　calculations　confirmed　that　the　electrons　are　energetically　favourable　to　transfer　toward　the　{10　(1)　over　bar0}　planes.　Upon　this　discovery,　PTI/Li+Cl-　crystals　with　different　aspect　ratios　were　prepared,　and　the　overall　water　splitting　performance　followed　a　linear　correlation　with　the　relative　surface　areas　of　the　{10　(1)　over　bar0}　and　{0001}　planes.　Our　controlling　of　the　reactive　facets　directly　instructs　the　development　of　highly　efficient　polymer　photocatalysts　for　overall　water　splitting.