During the past few decades, substantial efforts have been made to explore high-performance electrocatalysts for proton-exchange membrane fuel cells due to the sluggish oxygen reduction reaction kinetics at the cathode. Among multifarious approaches, it has been demonstrated that alloying Pt nanocrystals with transition metals and introducing Pt-based nanocrystals with high-index facets have been recognized as effective strategies to facilitate the enhancement of oxygen reduction reaction performance. However, the current synthetic approach for preparing catalysts with high-index facets is complicated. Herein, we demonstrate a straightforward one-step method to synthesize three-dimensional flower-like high-index faceted PtNi alloy nanocrystals constituted by abundant nano-cones similar to the quadrihedron. The as-prepared catalyst exhibits remarkably improved catalytic performance in comparison with PtNi-1, PtNi-2 and Pt/C, which is ascribed to a great number of high-index facets and highly open hierarchical structure. The resultant high-index faceted PtNi alloy nanocrystals deliver excellent mass activity of 1.76 A mg−1. Meanwhile, the material also displays a great stability after 20000 potential cycles. Consequently, the methodology proposed in this work may provide some guidance to boost the improvement of oxygen reduction reaction, and the 3D flower-like high-index faceted PtNi alloy nanocrystals have a great potential to be turned into effective cathode catalysts for proton-exchange membrane fuel cells.