Supported Pt-based alloy nanoparticles have attracted greater attention in catalysis due to their high activity, reduced cost, and easy recycling in chemical reactions. In this work, mesoporous SiO₂ microspheres were employed as support to immobilize PtNi alloy nanocatalysts with different mass ratios of Pt and Ni (1:0, 3:1, 1:1, 1:3 and 0:1) by a facile in situ one-step reduction in the absence of any capping agent. SEM, EDS, TEM, FTIR, XRD, ICP-AES, XPS and nitrogen adsorption/desorption analysis were employed to systematically investigate the morphology and structure of the obtained SiO2 microspheres and SiO₂/PtNi nanocatalysts. Results show that uniform PtNi nanoparticles can be homogeneously and firmly embedded into the surface of SiO₂ microspheres. When the as-prepared SiO₂/PtNi nanocatalysts were used in the reduction process of pnitrophenol to p-aminophenol, the nanocatalyst with Pt and Ni mass ratio of 1:3 showed the highest catalytic activity (TOF of 5.35 × 10¹⁸ molecules·g⁻¹·s⁻¹) and could transform p-nitrophenol to p-aminophenol completely within 5 min. The SiO₂/PtNi nanocatalyst can also maintain high catalytic activity in the fourth cycle, implying its excellent stability during catalysis.