In this study, numerical simulations of the behaviour of large-size sandstone specimens exposed to wood crib fires were conducted. The developed numerical model, which innovatively considers the reversible quartz expansion behaviour during heating–cooling cycles, can replicate the temperature distributions, cracking behaviours, and strength variations of sandstone samples under various thermal loads (e.g., fires). Results show that the cracking mechanisms during heating and cooling are different, and the spalling is due to their combining effects. During the heating process, tensile stresses are induced mainly inside the sample due to significant expansions of the outer layer. The induced tensile microcracks beneath the surface will be furtherly extended and widened due to the re-distributed local stresses caused by local failures. After the fire, significant tensile stresses appear in the outer part of the sample due to the increased thermal gradients (i.e., volumetric expansions) of the inner part, leading to crack widening on the surface. These widened cracks will coalesce with the heating-induced cracks in the earlier time and boost the fracturing and spalling behaviour until the temperature across the sample reaches an equilibrium state. The findings are essential for understanding the failure mechanism of sandstone during real fire development. The good agreements between lab tests and numerical simulation also reveal the feasibility of using numerical models to predict the fire-induced damage in sandstone elements in architecture.