Hierarchical nanowires (HNWs) exhibit unique properties and have wide applications, while often suffering from imperfect structute. Herein, we rep oft a facile strategy toward ultrathin CdS HNWs with monocrystal structure, where a continuous-wave (CW) Nd:YAG laser is,employed to irradiate an oleic acid (OA) solution containing precursors and a light absorber. The high heating rate and large temperature gradient generated by the CW laser lead to the rapid formation of tiny zinc-blende CdS nanocrystals which then line-up into nano:wires with the help of OA molecules. Nett, the nanowires experience a phase transformation from zinc-blende to wurtzite structure, and the transformation-induced stress creates terrates on their surface, which promotes the growth of side branches and eventually results in monocrystal HNWs with an ultrathin diameter of 24 nm. The one-step synthesis of HNWs is conducted in air and completes in just 40 s, thus being very simple and rapid: The prepared CdS HNWs display photocatalytic performance superior to their: nanoparticle counterparts, thus showing promise for catalytic applications in the future.