Hydraulic fracturing technology plays a key role in the efficiency exploitation of low-permeability oil-gas and geothermal reservoirs. In order to study the extension pattern of hydraulic fractures within the hot dry rock， the effect of flow rate and viscosity of fracturing fluid and horizontal geo-stress difference on the morphology of hydraulic fractures were investigated by the cohesive zone method （CZM）， and the combination of the above fracturing process parameters was optimized using orthogonal tests. The results show that the flow rate of fracturing fluid has a significant effect on the length of the hydraulic fracture， whereas the viscosity of the fracturing fluid has a significant effect on the width of the hydraulic fracture. The increase in the flow rate and viscosity of the fracturing fluid promotes the emergence and extension of branching fractures. When the horizontal geo-stress difference is 1MPa， the model established in this paper can obtain the best fracturing modification effect under the conditions that the fracturing fluid flow rate is 0.004m³/s and the viscosity is 0.07Pa·s； As the flow rate and viscosity of fracturing fluid exceed 0.004m³/s and 0.07Pa·s respectively and continuously increase， a decrease in the length and width of the hydraulic fracture will occur. Therefore， it is considered that the continuous improvement of fracturing effect cannot be achieved blindly by increasing the flow rate and viscosity of the fracturing fluid during the actual fracturing process. The findings of this study are expected to provide significant support in predicting the fracture extension behavior and the optimizing the fracturing parameters during the exploitation of hot dry rock geothermal resources.