In recent years, due to their exotic properties, wafer-scale synthesis of monolayer two-dimensional (2D) nanomaterials is vital for their practical utilizations in next-generation electronics and optoelectronics. Great efforts have been devoted in developing different synthesis techniques, such as various kinds of chemical vapor deposition (CVD), to obtain large-scale and uniform 2D nanomaterials. However, it is still a grand challenge to achieve the monolayer format with the large grain size and excellent uniformity across the entire wafer substrate. Herein, in contrast to other conventional growth methods, we demonstrate the wafer-scale synthesis (e.g. 2 inch in diameter) of highly crystalline, homogeneous and monolayer WS₂ by an enhanced CVD technique, in which precise control of precursor vapor pressures can be effectively achieved in the multi-temperature zone horizontal furnace. Importantly, these monolayers exhibit impressive optoelectronic properties and mechanical flexibility. When configured into typical rigid photodetectors, the monolayer WS₂ exhibits respectable photodetection performance with a responsivity of 0.52 mA/W, a detectivity of 4.9×10⁹ Jones and a fast response speed shorter than 560 μs. Furthermore, once fabricated as flexible photodetectors on polyimide, the photodetectors yield the responsivity of up to 5 mA/W. Also, the photocurrent can maintain 89% of its initial value even after 3000 bending cycles. All these results indicate the versatility of this technique to extend for larger substrates as well as the excellent mechanical flexibility and robustness of our large-scale, CVD grown, homogenous monolayer WS₂ for the advanced flexible optoelectronic devices.