Wurtzite-structured CdS material is widely used in information sensing and energy harvesting. Based on the piezoelectric property of CdS, we present a flexible piezoelectric nanogenerator (PENG) with three-dimensional-structured CdS nanowall arrays. Under index finger oscillations at a slow rate, the maximal open-circuit voltage and short-circuit current are 1.2 V and 6 nA respectively. Meanwhile, the working mechanism of this PENG was successfully studied with piezoelectric potential distribution and energy band theory respectively. All of the results show that an increase in the bending degree and bending frequency will affect the output of the PENG, suggesting that it can be used as a flexible sensor. In addition, the fabricated PENG can be used as a self-powered pressure sensor relying on the linear relationship between the output voltage and the vertical pressure. This work may provide a new approach to fabricating piezoelectric nanogenerators based on three-dimensional materials as an energy harvester, which may also facilitate the development of flexible and wearable electric sensing technology.