High-electron-mobility transistors (HEMTs) are a prom-ising device in the field of radio frequency and wireless communica-tion. However, to unlock the full potential of HEMTs, the fabrication of large-size flexible HEMTs is required. Herein, a large-sized (> 2 cm2) of AlGaN/AlN/GaN heterostructure-based HEMTs were successfully stripped from sapphire substrate to a flexible polyethylene terephtha-late substrate by an electrochemical lift-off technique. The piezotronic effect was then induced to optimize the electron transport performance by modulating/tuning the physical properties of two-dimensional elec-tron gas (2DEG) and phonons. The saturation current of the flexible HEMT is enhanced by 3.15％ under the 0.547％ tensile condition, and the thermal degradation of the HEMT was also obviously suppressed under compressive straining. The corresponding electrical performance changes and energy diagrams systematically illustrate the intrinsicmechanism. This work not only provides in-depth understanding of the piezotronic effect in tuning 2DEG and phonon properties in GaN HEMTs, but also demonstrates a low-cost method to optimize its electronic and thermal properties.