Piezoelectric nanogenerators that power micro/nano devices by converting surrounding tiny mechanical vibration into electrical energy and getting rid of batteries and power cables is attracting increasing attention in recent years. Piezoelectric nanocomposites combining the flexibility of polymers and piezoelectricity of nanostructures are the current research hot spot in this field. However, usually the piezoelectric constant (d33) of piezoelectric nanostructures cannot compete with those of ceramics, and that of lead-free nanostructures is even worse, leading to low output voltages and seriously restricting their applications. Here, we report a new piezoelectric nanocomposite based on 0.96(K0.48Na0.52)(Nb0.95Sb0.05)O3-0.04Bi0.5(Na0.82K0.18)0.5ZrO3 (KNNS-BNKZ) electrospun nanofibers with a ultrahigh d33 of 338 pm V-1 and significantly improved energy harvesting performance. Our KNNS-BNKZ nanofiber-based nanogenerator can generate an output voltage up to 10 V which is more than three times that of other reported lead-free piezoelectric nanocomposites. In addition, our nanogenerator can charge a capacitor up to 0.33 μF and 8 V in 45 seconds by hand-pressing after rectifying, showing its great potential in powering micro/nano electronic devices and sensors.