Achieving carbon neutralization by CO2 photoreduction (CO2PR) has attracted extensive attention. However, limited selectivity and harsh reaction conditions impede its practical application. Herein, we introduced a hydrogen storage material, ammonia borane (NH3BH3, AB), as mild proton–electron pair (adsorbed H atom, *H) donors, to realize synergistic hydrogen evolution and selective photocatalytic CO2 reduction to HCOOH. A preeminent HCOOH generation rate of 5237.8 μmol g–1 h–1 (with a HCOOH selectivity of 99.67% in carbonaceous products) was obtained. Intriguingly, *H provided by AB could accelerate selective HCOOH generation over Cu2O/TiO2 by promoting intermediate *HCOO generation in kinetics, and AB adsorption on Cu/TiO2 surface could also reduce the activation energy of HCOOH production in thermodynamics. Furthermore, some other similar *H donors have performed equally well in selective HCOOH, thus providing guidance for the design of a high-performance CO2PR system over the consideration of *H protons to meet the demand of sustainable development in the future.