Artificial photocatalysis offers a clean approach for producing H2O2. However, the poor selectivity and activity of H2O2 production hamper traditional industrial applications and emerging photodynamic therapy (PDT)/chemodynamic therapy (CDT). Here, we report a C5N2 photocatalyst with a conjugated C=N linkage for selective and efficient non-sacrificial H2O2 production both in normoxic and hypoxic systems. The strengthened delocalization of π-electrons by linkers in C5N2 downshifted the band position, thermodynamically eliminating the side H2 evolution and kinetically promoting the water oxidation. As a result, C5N2 had a competitive solar-to-chemical conversion efficiency of 0.55% in overall H2O2 production and exhibited by far the highest activity in hypoxic conditions (698 μM/h). C5N2 was further applied to hypoxic PDT/CDT with outstanding performance in apparent cancer cell death and synchronous bioimaging. It sheds light on carbon nitrides in photosynthesizing H2O2 for health.