The effect of WC nanoparticles on the spreadability, microstructure and mechanical properties of Cu/Sn57.6Bi0.4Ag/Cu solder joints was investigated. The results showed that the spreadability of Sn57.6Bi0.4Ag composite solder was enhanced by WC nanoparticles, and the spreadability was increased first and decreased with increasing WC content next; when the content of WC was 0.1 wt%, the solder got relatively optimal spreadability. The dispersed WC particles provided the heterogeneous nucleation sites during the soldering process and led to the microstructure refinement of Sn57.6Bi0.4Ag solder alloy. WC particles restrained the growth rate of IMC by reducing the surface energy of Cu6Sn5, the shear strength of Sn57.6Bi0.4Ag–xWC/Cu (x = 0, 0.1) solder joints decreased seriously with increasing aging time, WC particles improved the shear strength and toughness of Sn57.6Bi0.4Ag/Cu solder joints by refining the microstructure.