Compared with single metals, alloy nanocrystals (NCs) usually exhibit some novel properties and are emerging in the field of gas sensors. Uniformly dispersed PdCu NCs have been successfully synthesized and used to decorate W18O49 (same as WO2.72) nanowire array assembled from ultrafine nanowires with diameters less than 2 nm for hydrogen detection. The final Micro-Electro-Mechanical System (MEMS)-based PdCu-W18O49 gas sensor shows excllent selectivity and responsiveness to hydrogen at low operating temperature. The significantly improved sensing performance can be attributed to the following: 1) Ultrafine one-dimensional W18O49 nanowires could provide lots of adsorption sites and transport channels for hydrogen detection, 2) the content of Pd active sites increases with the introduction of the Cu metal, 3) the electronic sensitization of PdO, 4) the PdCu alloy NCs have excellent selective capability for hydrogen gas as they generate PdHx compounds, and 5) Pd4+ ↔ Pd2+ catalytic redox cycle accelerates the water formation reaction during hydrogen sensing. This work is of great significance for the detection of hydrogen, and has application potential in monitoring hydrogen leakage.