Thermal cracking of rock is a critical subject in high radioactive waste disposal engineering. A laboratory simulation test was conducted to study the thermal cracking of Beishan granite. The temperature measuring device, acoustic emission (AE) system, ultrasonic computer tomography(CT) and microscope were applied to the test. The results show that the macrocracks originated at the outer edges of the specimen and then extended inward. The growth of cracks was mutational, and the latter cracks were suspected little impact of the former cracks. Four stages were observed during the test: stable thermal damage stage, macrocrack formation stage, macro-crack growth and the crack closure stage during cooling period. The evolution of AE locations reveals the growth of cracks’ propagation process. The ultrasonic imaging of the cooled sample was obtained by the elastic wave CT. The cracks’ positions and area suffered heat damage could be visualized in the CT imaging. The study shows that system combined AE and ultrasonic CT is effective to characterize the evolution of thermal-cracking process and quantify the damage. Temperature field obtained from a group of thermal couples can be used to verify the parameter for numerical simulation. The results of numerical simulation show that the high tensile stress at the time of macrocrack appearing is consistent with the tensile strength of Beishan granite. Thermal induced damage and thermal-mechanical induced crack are analyzed systematically, so as to provide reference for the future related tests conducted in underground laboratory.