Seismic exploration is the most precise geophysical method for investigating and forecasting subsurface resources, yet its conventional fully elastic assumption neglects temperature effects, limiting its utility in geothermal studies. We present a novel full-waveform inversion (FWI) approach based on thermoelastic theory, enabling simultaneous inversion for subsurface temperature and elastic velocities by incorporating thermal influences into seismic wave propagation. The inversion framework minimizes the least-squares misfit between observed and calculated multi-component seismic data, with gradients for P-wave velocity, S-wave velocity, and temperature derived via the adjoint-state method. To address the nonlinearity and mitigate cycle-skipping, we progressively incorporate the multiscale inversion strategy into higher frequencies. Synthetic experiments on a modified Marmousi 2 model with coupled velocity and temperature anomalies demonstrate that thermoelastic FWI can accurately recover high-resolution velocity and temperature fields, closely matching the true models. These results confirm the feasibility of joint velocity-temperature inversion, establishing a theoretical foundation for direct seismic characterization of high-temperature geothermal reservoirs.