The fast development of wearable electronic systems requires a sustainable energy source that can harvest energy from the ambient environment and does not require frequent charging. Piezoelectric polymer films are a perfect candidate for fabricating piezoelectric nanogenerators (PENGs) to harvest mechanical energy from the environment due to their flexibility, good piezoelectricity, and environmental-independent stable performance because of their inherent polarization. However, most of their applications are limited to the pressing mode energy harvesting that is based on the 3-3-direction piezoelectric effect due to the molecular polarization and nonstretchability. In this work, by 3D printing an auxetic structure on a polymer film-based PENG, the bending deformation of the PENG can be transformed into the well-controlled in-plane stretching deformation, enabling the 3-1-direction piezoelectric effect. The synclastic effect of the auxetic structure is applied in flexible energy harvesting device for the first time, which makes the previously untapped bending deformation on a film a valuable device for energy harvesting and increases the bending output voltage of the PENG by 8.3 times. The auxetic structure-assisted PENG is also demonstrated as a sensor to sense the bending angle and monitor the motion by mounting on different joints of the human body and soft robotic finger.