In order to study the Brazilian splitting characteristics of sandstone under the coupling effect of high temperature and size effect, Brazilian splitting laboratory tests were carried out on standard sandstone specimens treated at 25 ℃, 200 ℃, 400 ℃, 600 ℃, 800 ℃ and 1000 ℃ respectively, and Brazilian splitting numerical simulation of high temperature sandstone with different sizes was carried out based on particle flow software to study the Brazilian splitting strength and deterioration law of sandstone Hysteresis law of porosity rise relative to crack propagation and penetration. The results show that: (1) In the temperature range of 25~1000 ℃ and the diameter range of 50~100 mm, the temperature and size effects have significant effects on the Brazilian splitting strength of sandstone. With the increase of temperature, the splitting strength of sandstone increases before 400 ℃ and continues to decrease after 400 ℃, and the splitting strength of each sandstone sample decreases by about 34.66%~35.10% in the whole process; With the increase of size, the splitting strength of sandstone decreases, and each sandstone sample decreases by about 55.61%~56.99%. In the range of values studied, the size effect has a greater influence on the Brazilian splitting strength of sandstone. (2) The relationship between the degradation amplitude of Brazilian splitting strength of sandstone and its diameter satisfies a negative exponential function, which can be used to predict the Brazilian splitting strength of different sizes of high-temperature sandstone, and has a certain reference significance for post disaster roof maintenance. (3) The porosity of sandstone increases during Brazilian fracturing, and the lag time relative to fracture propagation and penetration increases with the increase of temperature and size; considering the coupling effect of the two factors, the influence degree of size effect on lag time decreases with the increase of temperature, and the influence degree of temperature on lag time decreases with the increase of sandstone size. The research results can provide useful references for rock engineering design involving high temperature and size change, such as nuclear waste treatment, geothermal resource development and deep well engineering.