Rock mass is frequently subjected to rapid cooling in geothermal reservoir during water injection and reinjection. In this paper, to understand the effects of cooling treatments on heated granite, heat conduction tests, magnetic resonance imaging tests and numerical investigations were carried out to evaluate variations of thermal damage. The test results reveal that the heat flux and the heat transfer coefficient increases to a maximum within a few seconds and then gradually decreases. The maximum heat transfer coefficient of the samples treated with the initial temperature of 500, 400, 300, 200 and 100 °C is 2.3, 2.15, 1.9, 1.22 and 1.86 W·m⁻²K⁻¹, respectively. The edge area with drastic temperature changes is accompanied by the densely distributed microcracks; in contrast, the internal cracks of the specimen with gentle temperature are relatively sparse. The thermal damage contributed by the heating cracks occurs at a continuous decrease, and the thermal damage contributed by cooling occurs at a continuous increase, with the increasing heating temperature. The damage caused by heating is the result of the uneven thermal expansion of the local particles, the propagation of cooling cracks is strongly affected by heating cracks, and stress concentration induced by thermal shock promotes the coalescence of the pre-existing heating cracks.