Abstract: To study the Brazilian splitting characteristics of sandstone under the coupling effect of high temperature and sandstone’s size, Brazilian splitting laboratory tests were carried out on standard sandstone specimens treated at 25, 200, 400, 600, 800, and 1000 ℃, respectively. A Brazilian splitting numerical simulation of sandstone with different sizes under high temperature was carried out based on particle flow software to study the Brazilian splitting strength and deterioration law of sandstone. In addition, the hysteresis law of porosity rise relative to the crack propagation and penetration was also investigated. Results are as follows: (1) In the temperature range of 25−1000 ℃ and in the diameter range of 50–100 mm, the temperature and size significantly affect the Brazilian splitting strength of sandstone, with size having a greater influence. During the heating process, due to the initial thermal expansion in the rock and subsequent damage under the action of thermal stress, the splitting strength of sandstone first increases and then decreases by approximately 34.66%–35.10% after 400 ℃. With the increase in the size, the energy accumulated in the rock is released, and a large number of microfractures are produced, resulting in decreasing the splitting strength of sandstone samples by approximately 55.61%–56.99%. (2) The relationship between the degradation amplitude of the Brazilian splitting strength of sandstone and its diameter satisfies a negative exponential function, which can predict the Brazilian splitting strength of sandstone with different sizes at high temperatures. (3) The porosity of sandstone increases during Brazilian fracturing, and the load difference relative to fracture propagation and penetration increases with increasing temperature and size. Considering the coupling effect of the two factors, the influence of size and temperature on the load difference decreases with increasing temperature and sandstone’s size. This study is of high significance for roof maintenance and preliminary prediction of the roof strength after a fire. In addition, it can also provide a useful reference for rock engineering design involving high temperatures and size changes, such as nuclear waste treatment, geothermal resource development, and deep well engineering.