An integrated theoretical-experimental investigation is performed to understand the photocatalytic and optical properties of Ag/SrTiO3 nanocomposite (Ag/STO). Theoretical investigation reveals that the catalytic activity of Ag/STO is increased when Ag particle size is smaller, while the opposite correlation is observed for its visible-light absorbance efficiency. These insights suggest that efficient Ag/STO photocatalyst needs to be balanced between the active interfacial site density and visible-light absorbance intensity by carefully controlling the Ag dosage. Furthermore, reactive oxygen species that are responsible for the oxidative degradation of organic pollutant on Ag/STO could be identified from Density Functional Theory (DFT) calculations. Comprehensive experiments are carried out using Rhodamine-B (RhB) photodegradation to test the activity of Ag/STO in wastewater treatment application and excellently validate those theoretical predictions. Over series of synthesized Ag/STO composites with different Ag contents, the optimum 1 % wt. Ag loading has the highest 92.8 % efficiency in RhB photodegradation after 1 hour of light irradiation. Trapping experiments also confirm the crucial role of O2 and OH species, which was predicted from DFT calculations, as the primary oxidizing agents for the degradation of RhB. This work provides a useful framework to develop novel plasmonic nanocomposites for other photocatalytic applications.