Abstract: An atmospheric pressure plasma jet (APPJ) approach is developed to prepare platinum nanoparticles (PtNPs) under mild reaction conditions of lower temperatures and without adding chemical reagents. Optical Emission Spectroscopy (OES) and X-ray Photoelectron Spectroscopy (XPS) tests revealed that the APPJ contains a large number of high-energy active particles, which can generate solvated electrons in liquid thereby promoting the rapid reduction of Pt(IV) ions into Pt(0) atoms, and these atoms gradually grow into nanoparticles. After 3 min of treatment, PtNPs exhibit excellent dispersibility with a particle size distribution ranging from 1.8 to 2.8 nm. After 5 min, the particle size increases, and aggregation occurs. The zeta potentials for the two situations were −56.0 mV and −12.5 mV respectively. The results indicate that the treatment time has a significant impact on the dispersion, particle size distribution, and sol stability of the nanoparticles. Furthermore, it reveals the formation mechanism of PtNPs prepared by APPJ, which involves the generation and expansion of nanocrystalline nuclei, and the construction of negatively charged colloidal particles. The overall mechanism highlights the importance of the plasma-liquid interaction in the synthesis of PtNPs, offering a new perspective on the controllable production of nanomaterials using plasma technology.