Crack disease is widespread in stone cultural relics, and brings great challenges to their preservation. Therefore, it is important to carry out in-depth research on crack disease for the conservation of cultural relics. In this study, through the use of infrared thermal imaging technology, both indoor model tests and field test research for crack detection of stone cultural relics were carried out. The influence of the crack on the temperature field of rock mass was studied with the crack opening width being a main variable. The results show that 1) the active thermal excitation method was effective in crack detection; 2) the thermal inertia of the air in the fracture was large, causing the temperature change at the fracture to lag; 3) a cooling index was introduced in this study to quantitatively evaluate the temperature change rate of the sample, and had a good negative correlation with the fracture area ratio, which was verified by the field test results. These research results have preliminarily achieved the goal of quantitative detection of apparent cracks in stone cultural relics.