The complex response of rocks to high temperatures and mixed stress poses a significant challenge in deep rock engineering. Sandstone samples with varying notch offsets were subjected to thermal treatment and then tested under mixed mode I-II loading. The effect of notch offset and temperature on rock fracture mechanism is the focus. By employing digital image correlation (DIC) technology, the complete process of rock fracture was scrutinized and quantitatively characterized. The results indicate that thermal treatment has a negative effect on the peak load, fracture toughness, and fracture energy of sandstone. Specifically, sandstone samples treated at 700 °C had a fracture toughness that was less than half that of the samples tested at room temperature. Furthermore, the fracture toughness and fracture energy of sandstone exhibited significant increases with increasing notch offset. In particular, the crack tip displacement (CTD) and crack mouth displacement (CMD) curves of the samples with 72 mm notch offset, compared to the pure mode I loading, show a clear infection point near the peak load. The fracture process zone (FPZ) demonstrates an approximately linear increase with both temperature and notch offset. Additionally, thermal analysis was performed on the sandstone samples, revealing the impact of high temperature on the microstructure and mineral phase transition.