The performance of catalyst depends on the intrinsic activity of active sites and the structural characteristics of the support. Here, we simultaneously integrate single nickel (Ni) sites and platinum-nickel (PtNi) alloy nanoparticles (NPs) on a two-dimensional (2D) porous carbon nanosheet, demonstrating remarkable catalytic performance in the oxygen reduction reaction (ORR). The single Ni sites can activate the oxygen molecules into key oxygen-containing intermediate that is further efficiently transferred to the adjacent PtNi alloy NPs and rapidly reduced to H₂O, which establishes a relay catalysis between active sites. The porous structure on the carbon nanosheet support promotes the transfer of active intermediates between these active sites, which assists the relay catalysis by improving mass diffusion. Remarkably, the obtained catalyst demonstrates a half-wave potential of up to 0.942 V, a high mass activity of 0.54 A·mg_Pt⁻¹, and negligible decay of activity after 30,000 cycles, which are all superior to the commercial Pt/C catalysts with comparable loading of Pt. The theoretical calculation results reveal that the obtained catalyst with defect structure of carbon support presents enhanced relay catalytic effect of PtNi alloy NPs and single Ni sites, ultimately realizing improved catalytic performance. This work provides valuable inspiration for developing low platinum loading catalyst, integrating single atoms and alloy with outstanding performance in fuel cell.