Flexible thermoelectric generators (FTEGs) offer a promising solution for powering wearable electronics, while their practical applications are mainly obstructed by the moderate properties of flexible thermoelectric (TE) materials. Here, flexible Ag₂Se nanowire (NW)/methyl cellulose (MC) composite films were developed via facile screen-printing technology combined with cold pressing and annealing treatment, and a highest power factor of 1,641.58 μW m⁻¹ K⁻² at 360 K was achieved. The reasons for the high TE performance of the Ag₂Se NW/MC composite films were because, after the annealing treatment, the Ag₂Se NWs were sintered to form conductive network structures, the crystallinity of Ag₂Se was markedly enhanced, and the content of insulating phase MC in the composite film was decreased. The Ag₂Se NW/MC composite film held appreciable flexibility, as its room-temperature power factor (1,312.08 μW m⁻¹ K⁻²) can retain ~93% after bending for 1,000 cycles at a radius of 4 mm. Furthermore, the assembled FTEG consisting of 4 strips can generate a maximal power density of 3.51 W m⁻² at a temperature difference of 14.1 K. Our results open an effective and large-scale strategy for fabricating high-performance flexible TE materials and energy-harvesting devices.