Purpose <p>This study aims to analyze the seismic ground motion of a free field of viscoelastic unsaturated soil under SV wave incidence considering thermal-hydraulic-mechanical coupling effects, and to reveal the influence of parameters such as relaxation time and thermal expansion coefficient on the seismic response.</p> Methods <p>Based on the wave theory for unsaturated porous media, the governing equations incorporating soil skeleton viscosity and thermal effects are established by combining the Kelvin-Voigt viscoelastic model with generalized thermoelastic theory. For the case of plane SV-wave incidence, the analytical solution for the seismic response of the free field is derived using the Helmholtz decomposition method, and parametric numerical analysis is conducted.</p> Results <p>The results indicate that the amplification factors of horizontal displacement in the thermoviscoelastic model generally fall between those of the thermoelastic and viscoelastic models. The vertical displacement amplification factor increases with incident angle in both the thermoelastic and thermoviscoelastic models and exhibits a sharp rise near the critical angle. An increase in relaxation time leads to a decrease in the horizontal displacement amplification factor, while the vertical displacement amplification factor decreases first and then shows accelerated growth near the critical angle. An increase in saturation degree results in an increase in the horizontal displacement amplification factor but a decrease in the vertical displacement amplification factor. Variations in the thermal expansion coefficient cause the horizontal and vertical displacement amplification factors to first decrease and then increase, and first increase and then decrease, respectively. As the medium temperature rises, the displacement amplification factors initially increase within a specific range of incident angles and then stabilize, fluctuating within a certain range. The phase delay times of temperature gradient and heat flux lead to a distinctly banded distribution pattern in the displacement amplification factors.</p> Conclusions <p>For the seismic response of unsaturated soil in a free field under SV-wave incidence, mechanical parameters and viscoelastic properties are the dominant controlling factors, whereas the influence of thermophysical parameters is comparatively weak and can be regarded as secondary.</p>