As soft robotics fast advances, full autonomy becomes highly sought, especially if their motion can be powered by environmental energy and self-regulated. This would present a self-sustained fashion in terms of both energy supply and motion control. Currently, autonomous movement can be realized by leveraging out-of-equilibrium oscillatory motion of stimuli-responsive polymers under a constant light source. It would be more advantageous if environmental energy can be scavenged to power robots. However, generating oscillation becomes challenging under the limited power density of available environmental energy sources. Herein, we develop fully autonomous soft robots with self-sustainability based on self-oscillation. Aided by multiphysics modeling, we have successfully reduced the required input power density to around one-Sun level through a liquid crystal elastomer (LCE)-based bilayer structure. The autonomous motion of the low-intensity LCE/elastomer bilayer oscillator “LiLBot” under low energy supply was achieved by high photothermal conversion, low modulus, and high material responsiveness simultaneously. The LiLBot features tunable peak-to-peak amplitudes from 4 degrees to 72 degrees and frequencies from 0.3 Hertz to 11 Hertz. The oscillation approach offers a general strategy for desgining autonomous, untethered and sustainable small-scale soft robots, such as sailboat, walker, roller, and synchronized flapping wings.