Hierarchical Pt-alloys enriched with active sites are highly desirable for efficient catalysis, but their syntheses generally need time-consuming and elaborate annealing treatment at high temperature. We herein report a surface active-site engineering strategy for constructing the hierarchical PtNi nanocatalysts with an atomic Pt-skin layer (PtNi@Pt-SL) towards efficient triiodide reduction reaction (TRR) via an acid-dealloying approach. The facile acid-dealloying process promotes the formation of surface Pt active sites on the hierarchical Pt-alloys, and thus results in good catalytic performance towards TRR. Theoretical calculation reveals that the enhanced catalytic property stems from the moderate energy barriers for iodide atoms on the surface Pt active-sites. The surface active-site engineering strategy paves a new way for the design of active and durable electrocatalysts.