MoS2 has attracted tremendous attention as a substrate for supporting noble metal nanoparticles profiting from its ability to spontaneously reduce noble metal ions into nanoparticles. However, little is known of the mechanism and behavior of such spontaneous reduction. In this work, observation of Au3+ reduction on MoS2 is performed using atomic force microscopy (AFM) to obtain a better understanding of the reduction mechanism and behavior. AFM, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) confirm that Au3+ could be well reduced into Au nanoparticles when MoS2 serves as a substrate. No oxidation of MoS2 is observed during the reduction of Au3+, suggesting that the oxidation of MoS2 is not the driving force for the reduction. AFM and XPS demonstrate that the reduction is a light-induced reaction. MoS2 would release free photogenerated electrons under light irradiation, which are the electrons involved in the reduction reaction and lead to the reduction of Au3+ into Au nanoparticles. AFM further reveals that light intensity, near-ultraviolet light, and temperature promote the reduction of Au3+. In addition, Au nanoparticles prefer to assemble along the edges of MoS2. The findings in this work could give insights into the control and growth of noble metal nanoparticles on the MoS2 substrate for producing better composites for photocatalysis and surface enhanced Raman scattering sensing.