The　solar　H-2　generation　directly　from　natural　seawater　is　a　sustainable　way　of　green　energy.　However,　it　is　limited　by　a　low　H-2　generation　rate　even　compared　to　fresh　water.　In　this　report,　TiO2　is　chosen　as　a　model　photocatalyst　to　disclose　the　critical　factor　to　deteriorate　the　H-2　generation　rate　from　seawater.　The　simulated　seawater　(SSW),　which　is　composed　of　eight　ions　(Na+,　K+,Ca2+,　Mg2+,　Cl-,　Br-,　SO42-,　and　CO32-),　is　investigated　the　effect　of　each　ion　on　the　H-2　production.　The　results　indicate　that　all　ions　have　a　negative　effect　at　the　same　concentration　as　in　the　seawater　except　Br-.　The　CO32-　has　the　most　serious　deterioration,　and　the　H-2　production　rate　lowers　near　40%　even　at　[CO32-]　of　1.5　mmol-　L-1.　The　H-2　production　rate　can　be　recovered　to　85%　if　the　CO32-　is　excluded　from　the　SSW.　To　understand　the　reason,　the　zeta　potential　of　the　TiO2　treated　with　different　ions　aqueous　solution　reveals　that　the　zeta　potential　decreases　when　it　is　treated　with　CO32-　and　SO42-　due　to　they　can　adsorb　on　the　surface　of　TiO2　nanoparticles.　Fourier　transform　infrared　(FTIR)　and　thermogravimetric　analysis-mass　spectroscopy　(TGA-MS)　further　confirm　that　the　adsorbed　ion　is　mainly　from　CO32-.　Since　the　pH　of　seawater　is　about　8.9　between　pK(a1)　(6.37)　and　pK(a2)　(10.3)　of　H2CO3,　the　CO32-　should　exist　in　the　form　of　HCO3-　in　the　seawater.　We　proposed　a　simple　method　to　remove　the　adsorbed　HCO3-　from　the　TiO2　surface　by　adjusting　the　pH　below　the　pK　al　.　The　results　indicate　that　if　a　trace　amount　of　HCI　(adjusting　pH　similar　to　6.0)　is　added　to　the　SSW,　the　H-2　production　rate　can　be　recovered　to　85%　of　that　in　pure　water.