Abstract: Hot dry rock (HDR) formations are characterized by four high conditions: high temperature, high hardness, high in-situ stress, and a large differential between the two horizontal stresses. These extreme conditions present several technical challenges, including high breakdown pressures, difficulty in forming complex fracture networks, small stimulated reservoir volumes, and a risk of inducing strong micro-seismic events. As a result, existing unconventional oil and gas fracturing techniques cannot be directly applied to HDR reservoirs. To address these challenges, a series of physical simulation experiments were conducted on large-scale HDR outcrop rock samples. The effects of different geological and engineering conditions on fracture initiation pressure, fracture propagation complexity, and the number and energy level of induced micro-seismic (acoustic emission) signals were investigated. The results show that hydraulic fracturing in HDR first activates natural fractures and then extends along those pre-existing fractures. Injecting CO₂ as the fracturing fluid was found to reduce the breakdown pressure by about 30% and decrease the acoustic emission energy level by roughly 15%. In addition, cyclic injection with variable flow rates readily triggers and induces the development of a more complex fracture network. These findings indicate that natural fracture presence, large thermal differential effects (due to cold fluid injection into hot rock), and alternating injection of slickwater and supercritical CO₂ are the primary controlling factors for creating complex fractures in HDR reservoirs. Based on these insights, a flexible reservoir stimulation technique for HDR is proposed, which integrates three goals into one approach: lowering the fracture initiation pressure, enhancing fracture network complexity, and preventing the occurrence of strong induced micro-seismic events. Field trials of this flexible stimulation method in areas such as Qinghai, South China, and Jiangsu have yielded promising results. This approach has positive implications for advancing volumetric reservoir stimulation techniques in hot dry rock geothermal systems.