With a rapid expansion in the field of wearable electronics, powering them entirely by batteries has become more and more unpractical and unfavorable. Here, we developed a lightweight, flexible and sustainable power source by fabricating a nanofibrous membrane constructed wearable triboelectric nanogenerator (NM-TENG), which is capable of converting human biomechanical energy into electricity for next-generation wearables. With an effective device area of 16cm2 under gentle hand tapping, it can deliver a current and voltage output respectively up to 110µA and 540V. And the electrospun nanofibrous membranes were tailored to enhance the triboelectric polarity, mechanical strength as well as surface hydrophobicity, which will eventually improve the device output performance, robustness and capability of operation even with high environmental humidity. Via harvesting the biomechanical energy from body motion, the wearable NM-TENG was demonstrated to sustainably power a commercial thermal meter, electronic watch, and light up about 560 LEDs. Given a collection of compelling features of being flexible, breathable, environmentally friendly and cost-effective, the NM-TENG can be extensively applied not only to self-powered wearable electronics but also possibly to power generation at a large scale.