Strain-sensing capabilities are fundamental features for building the next generation of artificial intelligent devices. However, the development of sensors with such functionality has been hindered by the requirements of external power sources and the need for devices with long cycle life and high resolution. Here, we report a self-powered ultrasensitive strain sensor system based on a graphene-ecoflex nanocomposite film, combining good thermoelectric property of graphene and large tensile property of ecoflex elastomer. The device maintains excellent stability with repeatable output signals over 1000 cycles under repetitive strains from 0% to 100%, and exhibits a strain resolution of 0.125% with a response time of less than 0.6 s. As a wearable device attached to the human skin, it can precisely detect subtle human motions based on the temperature gradient between the human skin and ambient environment. A multi-channel 3D strain sensor system was further manufactured to feature the function of strain detection along various directions.