GaN high-electron-mobility-transistors (HEMTs) exhibit superior high-power and high-frequency characteristics; however, they generate a substantial amount of heat during operation. As quantum piezotronics devices, GaN HEMTs are of particular interest due to their strong coupling processes between piezoelectric, electrical and thermal fields, which are still being explored. For the first time, we built a theoretical framework combing piezotronics with the electrothermal model to reveal the thermal spatial distribution and temporal evolution. Advanced infrared thermography exhibit heat source is localized near the gate in GaN HEMT, which matches our theoretical model. The dynamic temperature characteristics indicate that the substrate layer contributes to the main thermal resistance and capacitance. Notably, through introducing external stress, piezoelectric polarization can be a probe to locally modulate thermal fields. A 10.1% decrease in temperature-rise is realized during the dynamic modulation process, which further confirmed accuracy of the model. This work deepens understanding and cognition of piezoelectric-electric-thermal coupling processes and guides novel thermal management strategy in GaN HEMT device.