Based on true triaxial simulation experimental system, servo control system of hydraulic fracturing equipment and tracer technique, a large-scale true triaxial physical simulation test on hydraulic fracturing of shale horizontal well is conducted. The extension of the hydraulic fractures is analyzed by dynamically monitoring of fractures and sectioning the fractured specimen. The formation mechanism of fracture network is discussed. The results indicate that: (1) A relatively complex fracture morphology is formed due to the hydraulic fractures extending along and perpendicular to the bedding planes after initiation. (2) The fracture network which contains longitudinal and transverse fractures is formed in hydraulic fracturing of shale specimens. Main hydraulic fractures perpendicular to the bedding plane and secondary fractures extending along the week bedding plane all exist in the fracture network. (3) Hydraulic fractures will gradually turn to the direction perpendicular to the minimum principal stress in extension. (4) Arresting, branching, penetrating and re-orientation of hydraulic fractures in bedding planes are the main reason for the formation of complex fracture network of shale gas reservoir. The existance of the weak structural planes is the foundation of the forming of complex fracture morphology. The results can provide technical supports for staged exploitation of horizontal well of shale gas reservoir.