Harvesting ambient mechanical energy is a crucial technology for realizing self-powered electronics, which are widely used in wireless sensing networks, implantable devices, and portable electronics. However, mechanical motions in the environment are random, low-frequency, and limited in amplitude, making it challenging to recycle these energies efficiently and stably. Herein, inspired by counter-rotating propellers, a triboelectric nanogenerator based on a coaxial counter-rotating unit (CCR-TENG) was designed and fabricated. Specifically, the CCR-TENG consists of a coaxial counter-rotating unit and a free-rotating disk triboelectric nanogenerator (FRD-TENG). The coaxial counter-rotating unit plays a primary role in converting random ambient mechanical motions into high-speed relative rotation motion. Accordingly, the generated electronic energy of CCR-TENG is improved by more than 15 times compared with conventional devices through the efficient motion mode conversion. Moreover, by introducing a soft nylon fur layer into the FRD-TENG, the surface triboelectric charges are replenished in soft contact mode under the mechanical excitation. Thereby, the output performance and durability of the CCR-TENG are further improved. Remarkably, 180 LEDs and three typical portable electronics were continuously powered using CCR-TENG to harvest energy from random human motions. This work provides a promising strategy for the efficient and stable harvesting of ambient mechanical energy.