The synthesis of atomically precise semiconductors Ag-S clusters is a subject of intense research interest, yet the formation mechanism of such nanoclusters remains obscure due to their uncontrolled fast nucleation process in solution. Herein, we have investigated the reaction mechanism responsible for {Ag32S3} nucleation using UV, ESI-MS, NMR and SCXRD analyses. Triphenylphosphorus sulfide (PPh3S) was surprisingly found to slow down the kinetic process of the cluster nucleation. Furthermore, a key precursor [Ag2(Ph3PS)4]2+ was captured, which was attacked by Agn(CCBut)m and traces of water to generate {Ag32S3}. This mechanism provides valuable new insights into the synthesis of inorganic magic-size clusters.