Plasmonic AuPd alloy nanoparticles supported on super small carbon nitride nanospheres (AuxPdy/CNS) for the design of Mott-Schottky catalysts were successfully synthesized and further applied for the photocatalytic hydrogen evolution from formic acid. A high turnover frequency (TOF) value of 1017.8 h(-1) was obtained for the AuPd/CNS catalyst under visible-light irradiation (lambda > 420 nm) at 298 K. XPS analysis, photoelectrochemical characterization and density functional theory (DFT) calculation indicate that the remarkable photocatalytic activities are mainly attributed to the optimized electronic structure of Pd in the AuPd/CNS composite resulting from the alloying, plasmonic and Mott-Schottky effects. These effects can efficiently accelerate the electron transfer from photoresponsive super small carbon nitride nanospheres and plasmonic Au to the active Pd sites. We also infer that the alloying effect is the main factor on the high activity, which is mainly due to weakened adsorption of hydrogen atoms on Pd sites according to the DFT calculation. Moreover, the Mott-Schottky AuPd/CNS catalyst presents a good universality for the photocatalytic hydrogen evolution from a series of aldehyde aqueous solutions.