Abstract: To understand the mechanism of hydraulic fracture initiation and propagation in deep hot-dry-rock reservoirs, a THM coupling model was established based on the phase field method. The study of hydraulic fracture expansion behavior in hot-dry-rock reservoirs was carried out. The effects of temperature, pump rate, and natural fractures on the hydraulic fracture expansion rule was analyzed. The established model based on phase field method has a simple criterion for fracture initiation. The calculation is accurate. In the process of hydraulic fracturing, thermal stress appears as tensile stress, which plays an auxiliary role for fracture propagation and increases the fracture width. The larger the temperature difference between fracturing fluid and formation, the larger the pump rate, and the more significant the effect of thermal stress. It is easy for hydraulic fractures to communicate with natural fractures after encountering natural fractures. When hydraulic fractures crack again, they crack along the direction of natural fractures and deflect to the direction of horizontal maximum principal stress under the action of geo-stress field. The research results can provide reference for the hydraulic fracturing in deep HDR reservoirs.