Photocatalytic water splitting for hydrogen energy is one of the most promising ways to solve the energy crises. The mechanism is unclear on the sequence of proton coupled electron transfer (PCET) in photocatalytic water dissociation catalyzed by organic material. Here, the water splitting catalyzed by zinc porphyrin with boron dipyrrin (ZnPP-BDP) is systematically investigated by density functional theory (DFT) calculations. The H2O/ZnPP in valence state based on the electron transfer pulls the trigger on the water splitting process. For the oxygen evolution reaction (OER) step on ZnPP, the processes for the two H ions dissociate from water are exothermic with the −1.06 and −0.95 eV energies respectively, and the O/ZnPP system is formed. The second H2O molecule on O/ZnPP system can provide the extra oxygen atom to produce the free O2 with 1.22 eV energy barrier. For the hydrogen evolution reaction (HER) step on BDP, the H ions dissociated from OER process can be captured by BDP to form free H2 with 2.24 eV/molecule energy released. In this attractive clean cyclic process, the ZnPP-BDP can continuously catalyze the H2O dissociated into free H2 and O2 by the shifting potential barrier. Besides, the proton coupled electron sequent transfer possess an essential role in photocatalytic water dissociation. It is very instructive to design sustainable photodecomposition catalysts for both OER and HER, and to design extraordinary biomimetic photosynthesizers for clean energy. Display omitted •Biomolecule ZnPP-BDP can split water into free H2 and O2 completely.•Proton coupled electron sequent transfer is vital in photocatalytic water splitting.•The water splitting catalyzed by ZnPP-BDP is sustainable and clean.•The effective catalysis is due to the effect of potential barrier transfer.