The　performance　of　proton　exchange　membrane　fuel　cells　is　heavily　dependent　on　the　microstructure　of　electrode　catalyst　especially　at　low　catalyst　loadings.　This　work　shows　a　hybrid　electrocatalyst　consisting　of　PtNi-W　alloy　nanocrystals　loaded　on　carbon　surface　with　atomically　dispersed　W　sites　by　a　two-step　straightforward　method.　Single-atomic　W　can　be　found　on　the　carbon　surface,　which　can　form　protonic　acid　sites　and　establish　an　extended　proton　transport　network　at　the　catalyst　surface.　When　implemented　in　membrane　electrode　assembly　as　cathode　at　ultra-low　loading　of　0.05　mg(Pt)　cm(-2),　the　peak　power　density　of　the　cell　is　enhanced　by　64.4%　compared　to　that　with　the　commercial　Pt/C　catalyst.　The　theoretical　calculation　suggests　that　the　single-atomic　W　possesses　a　favorable　energetics　toward　the　formation　of　*OOH　whereby　the　intermediates　can　be　efficiently　converted　and　further　reduced　to　water,　revealing　a　interfacial　cascade　catalysis　facilitated　by　the　single-atomic　W.　This　work　highlights　a　novel　functional　hybrid　electrocatalyst　design　from　the　atomic　level　that　enables　to　solve　the　bottle-neck　issues　at　device　level.