Abstract: Due to the high stress accumulation in the rock bridge of its locked section, the rock slope with joints is prone to sudden instability. The uniaxial tests are carried out on the granite specimens with different rock bridge lengths. By full use of infrared camera monitoring, the thermal infrared images and temperature variation characteristics before rock burst are studied, and the acoustic emission and high-speed cameras are used to analyze the crack propagation sources, crack propagation trend, acoustic emission characteristics and evolution mechanism. It is discovered from the experimental study that there are two types of thermal infrared precursors before fracture of the rock bridge specimens, anomalous thermal image precursor and thermal infrared ones, and with the increase in the length of the rock bridges, the thermal infrared precursors become more prominent. The variation in thermal images at the tip of the lower prefabricated joints can be used to predict future crack propagation directions and to locate the origin of surface cracks. Using the strain energy density factor theory of fracture mechanics, the influence mechanism of rock bridge lengths on initiation crack propagation under uniaxial stress is revealed. The thermal infrared precursors are more reliable than other precursors by using the infrared thermal imager to monitor the variation characteristics of rock bridge specimens before fracture. The research results may provide a theoretical basis for the prevention and control of jointed rock slopes.