Stretchable strain sensors play an increasingly important role in artificial intelligent devices. However, high-performance strain sensors have been slowly developed owing to the harsh requirement of self-powered function, long cycle life and high resolution. Here, we report a self-powered stretchable graphene-ecoflex composite strain sensor based on photo-thermoelectric (PTE) effect induced electricity. The device exhibits a high strain sensitivity of -0.056 ln(nA)/% with strains ranged from 0% to 20% under 980 nm light illumination, where the strain sensitivity can be found to increase with increasing light intensity. The strain sensor maintains outstanding dynamic stability under periodic strains ranged from 0 to 100% in 100 cycles. The sensing resolution can be as high as 0.5% with both the response and recovery time of less than 0.6 s. It can precisely monitor human joint motions and stretchable strains by implanting the device in pork.