Visible light-driven H2O2 production presents the unique merits of sustainability and environmental friendliness. The size of noble metal nanoparticles (NPs) determines their dispersion and electronic structure and greatly affects their photocatalytic activity. In this work, a series of sized Au NPs over C3N4 were modulated for H2O2 production. The results show that there is a volcanic trend in H2O2 with the decrease of Au particle size, and the highest H2O2 production rate of 1052 μmol g-1 h-1 is obtained from medium-sized Au particles (∼8.7 nm). The relationship between structure and catalytic performance is supported by experimental and theoretical methods. (1) First, medium-sized Au NPs promote photon absorption, and have a suitable built-in electric field at the heterojunction, which can be successfully tuned to achieve a more efficient h+-e- spatial separation. (2) Second, medium-sized Au NPs enhance O2 adsorption, and create selective 2e- O2 reduction reaction sites. (3) Particularly, medium-sized Au NPs promote the desorption of produced H2O2 and inhibit H2O2 decomposition, finally leading to the highest H2O2 selectivity. Excellent catalytic performance will be obtained by finely optimizing the particle size in a certain range. This work provides a new idea for preparing high efficiently photocatalysts for H2O2 production.