The formation energies, stable configurations, electronic and magnetic properties of the Pt4 (the 4-atom Pt cluster) clusters adsorbed nearby a single-vacancy (SV) defect on a graphene surface were investigated using the first-principles density-functional theory method. It is found that on the pristine graphene the base atoms of the Pt4 cluster prefer to be adsorbed at the bridge or top sites of the carbon atoms. On the defective graphene with a single-vacancy, the Pt4 cluster prefers to be trapped at the defective site with enhanced formation energies and distorted structures. The graphene substrate plays a direct role in regulating the formation, electronic structure and magnetic properties of the Pt4 clusters. The catalytic activity of the adsorbed Pt4 clusters was also studied through adsorption of a probe molecule CO.