We carry out a numerical study on the thermal behaviour of asteroid 3200 Phaethon, the parent body of the Geminid meteor shower. Phaethon's orbit is highly eccentric, with the perihelion distance as small as [Formula: see text]. During the perihelion passage the surface temperature rises so much that the reaction force of the thermal radiation from its surface affects Phaethon's orbital and spin motion. We evaluate this thermal effect precisely by numerically modelling the thermal variation during an orbital period. We find that the thermal effect, both on the orbital motion and spin, changes its sign near the perihelion owing to the geometric configuration of the spin axis direction relative to the Phaethon-Sun direction. Consequently, the canonical expression of the thermally induced momentum is not applicable for the case of bodies on a highly eccentric orbit such as Phaethon. Nevertheless, the resulting orbital-averaged thermal effect is consistent with ground-based observation and previous estimates.This article is part of the theme issue 'Major advances in planetary sciences thanks to stellar occultations'.