Thermally induced microcracks alter the mechanical properties of rocks. Mode I loading prevails in nature, while rocks are typically weak in tension. Understanding the effects of pre-existing thermal microcracks on the microcracking mechanisms of granite under mode I loading is of both scientific and practical value. We conducted three-point bending tests on pre-notched semi-circular bend Hong Kong granite specimens, in which the acoustic emissions (AEs) were monitored. To introduce the thermal microcracks, the specimens have been thermally treated to different temperature levels (50 °C, 100 °C, 150 °C, 200 °C, and 400 °C) before the loading tests. The microcracking mechanisms are analyzed by deciphering the AE signals recorded from the start of loading until the macrocrack initiation. We find that the presence of the thermal microcracks aggravates the microcrack damage. Our detailed AE analysis reveals that the thermal microcracks do not significantly affect the proportions of three types of AE events (i.e. tensile, shear and mixed-mode) as well as their energy distributions. The tensile events dominate and the proportions of the three types are independent of event magnitude. The temporal evolution features of the event-type ratio are different between the specimens with few thermal microcracks and those with substantial thermal microcracks. For the former, the tensile event ratio rapidly increases to its peak, while the shear and mixed-mode event ratio decreases, which becomes a signature precursor prior to the initiation of a macrocrack. In contrast, this precursor is absent for the latter.