Brazilian splitting acoustic emission (AE) tests were conducted on the granite specimens that were heated at the temperatures from 50 to 600 °C to investigate the mechanical and damage characteristics of granite under high temperatures and splitting load in deep drilling. The results indicate that temperature variations significantly alter the mechanical characteristics and AE properties of granite, thereby influencing the evolution of fractures. At the temperature above 500 °C, the tensile strength of granite decreases significantly and the proportion of shear-induced damage modes increases. Furthermore, the rate of fracturing also exhibits a noticeable increase. Damage parameter mutation points on the damage parameter evolution curves of hit counts and energy counts were defined to determine the rock damage precursor. Mutation points based on hit counts arrive earlier than rock instability signals. The greater the damage is, the earlier the mutation points arrive. This time difference can be used to identify rock destabilization signals.