In　the　synthesis　of　highly　crystalline　carbon　nitride,　alkali　metal　halides　are　used　as　high-temperature　solvents　and　structural　guiding　agents,　but　the　effect　of　alkali　metal　ions　on　the　catalytic　activity　of　materials　is　still　unclear.　In　addition,　the　shape　and　size　of　the　cocatalyst　also　significantly　affect　the　catalytic　activity　of　the　material.　Therefore,　we　have　carried　out　the　density　functional　theory　calculations　to　reveal　the　differences　in　the　structure,　electronic,　and　optical　properties,　as　well　as　the　photocatalytic　hydrogen　evolution　performance　of　one　kind　of　crystalline　carbon　nitride　called　potassium　poly(heptazine　imide)　(K-PHI)　and　its　proton-exchanged　counterpart　(H-PHI)　loaded　with　different　sizes　of　platinum　cocatalysts.　The　results　show　that　potassium　ions　have　the　effect　of　aggregation　electrons　in　K-PHI,　and　potassium　ions　have　short-range　induced　dipole　effect　on　nearby　Pt　atoms,　resulting　in　slightly　better　photocatalytic　hydrogen　evolution　activity　at　these　reaction　sites.　Additionally,　Pt　cocatalyst　with　larger　load　size　has　better　photocatalytic　hydrogen　evolution　activity　than　small　size.　This　work　provides　a　theoretical　basis　for　the　rational　design　of　high-performance　metal　cocatalysts　and　exploration　of　the　functionality　of　alkali　metal　ions　in　carbon　nitrides.