By a facile water evaporation process without adding any directing agent, Cu2-xSe nanowire bundles with diameters of 100=300 nm and lengths up to hundreds of micrometers, which comprise crystalline nanowires with diameters of 5-8 nm are obtained. Experiments reveal the initial formation/ stacking of CuSe nanoplates and the subsequent transormation to the Cu2-xSe nanowire bundles. A water evaporation-induced self-assembly (WEISA) mechanism is proposed, which highlights the driving force of evaporation in promoting the nanoplate stacking, CuSe-to-Cu2-xSe transformation and the growth/bundling of the Cu2-xSe nanowires. The simplicity, benignancy, scalability, and high-yield of the synthesis of this important nanowire material herald its numerous applications. As one example, the use of the Cu2-xSe nanowire bundles as a photoluminescence-type sensor of humidity is demonstrated, which shows a good sensitivity, ideal linearity, quick response/recovery and long lifetime in a very wide humidity range at room temperature.