Controlling the nucleation and growth process of noble metal on surface of catalyst supports is one of the most effective strategies to improve the catalytic performance of supported noble metal catalysts. In the present study, a novel and green route was developed to in-situ load platinum (Pt) onto tin oxide (SnO2) nanocrystals. Initially, nonstoichiometric SnOx colloidal nanoparticles (NPs) with high reactivity were facilely prepared by laser ablation of a metallic Sn target in deionized water. During the subsequent aging process, the PtCl42- ions react with these SnOx NPs, which gradually evolved into stoichiometric SnO2. The obtained ultrafine Pt NPs together with large amounts of Pt clusters and single atoms were uniformly dispersed on the surface of SnO2 nanocrystals. For hydrogenation of 4-nitrophenol (4-NP), these Pt/SnO2 nanocomposites (NCs) show a significantly higher value of mass-normalized rate constant (k/m, 0.557 s(-1) mg(-1)) than that of most reported Pt-based catalysts, revealing remarkable catalytic activity. Also, no obvious deactivation was detected after seven successive cycles, indicating excellent catalytic stability of the Pt/SnO2 catalysts.