Band structure engineering is an effective strategy for the improvement in thermoelectric performance, especially in electrical transport properties. In this work, high pressure is employed to assist Te doping to rapidly realize modulation of band structure in BiCuSe_1-xTe_xO, and then achieving a superhigh carrier mobility of 129.6 cm²V^–1s^–1 due to significant reduction in the effective mass. The experimental observations have been verified by density functional theory (DFT) simulation. Meanwhile, the implementing of high pressure during synthesis process extends the optimization effect of Te doping on carrier-phonon transport of BiCuSeO system. The multiscale microstructures induced by synergistic effect of high pressure and Te content markedly modulate the scattering mechanisms of carriers and phonons, yielding an ultralow thermal conductivity of 0.3 W m^–1K^–1 at 873 K and a moderate effect on low-energy carriers. Ultimately, a maximum zT of 0.86 at 873 K is achieved for BiCuSe_0.8Te_0.2O, ~21% improvement in comparison with the previous reported value for state-of-the-art BiCuSe_1-xTe_xO samples. This study provides a revelation for employing high pressure to manipulate band structure, promoting the effect of heteroatoms doping on the improvement in thermoelectric performance of the BiCuSeO or other systems.