Polystyrene microspheres coated with modified Ag nanoparticles (PS/mAgNPs) were facilely fabricated via the proposed thermodynamically driven heterocoagulation method, and then their catalytic property was investigated using the reduction reaction of 4-nitrophenol as a model system. The surface modification of Ag nanoparticles (AgNPs) with less gold can subsequently lead to significant improvements in not only the coverage of the PS substrate with AgNPs but also the catalytic efficiency of the resultant composite catalyst. Furthermore, ammonia was found to have an evident cooperative effect with AgNPs in the catalytic reaction, as the reaction rate distinctly increased after introducing a trace amount of ammonia into the system.