Sub-nanometer-precision of noble metal catalysts with atomic contact to support simultaneously played a pivotal role in determining their catalytic activities. However, achieving predictable construction of noble metal cocatalysts at sub-nanometer scales remained a significant challenge. Herein, we demonstrated an in situ lattice atom abstraction strategy for sub-nanometer-precision construction of size-controlled Ag cocatalysts. Host lattice Ag⁺ was abstracted by tri-n-octylphosphine (TOP) due to its strong complexing ability towards Ag⁺, and was in situ reduced into metal Ag cocatalysts on the surface of AgInS₂ nanorods (AIS NRs), which was accelerated by the synergistic effect of TOP and oleylamine (OAm). This in situ lattice atom abstraction strategy avoided the undesired cation exchange reaction and simplified complex reaction processes, facilitating Ag cocatalysts with controlled sizes ranging from 0.60 to 6.76 nm with an unprecedented sub-nanometer precision. This set of Ag cocatalysts with sub-nanometer precision provided an ideal platform for systematically investigating cocatalyst size effects. Nano-sized Ag cocatalysts possessed superior separation and transfer ability over cluster-sized Ag cocatalysts, leading to the enhancement of photocurrent density 2.78 times higher than cluster-sized Ag cocatalysts. While cluster-sized Ag cocatalysts possessed higher surface catalytic activity, contributing to the improvement of Faradaic efficiency up to 97.5% from 74.4%.