Hydraulic fracturing technology is an important method to increase oil and gas production. In reservoirs such as shale and sandstone，different degrees of natural fractures develop. When hydraulic fracturing is used for fractured reservoirs，the existence of natural fracture has a significant impact on the formation and development degree of hydraulic fracture，especially for fracture network. In order to analysis the rock failure process，a two-dimensional plane-strain calculation model based on the FEM（finite element method）was established considering the development of natural fractures.The formation process of hydraulic fractures and complex fracture network in the hydraulic fracturing process were researched，and its influencing factors were analyzed. Grid-coverage method based on box counting method was introduced to measure the fractal dimension of the complex reservoir fractures and the fracturing effect obtained by numerical simulation was evaluated. The simulation results suggest that the development of natural fractures definitely affects the produced fractures.When propagating near natural fractures，hydraulic fractures will bifurcate，slip along the natural fracture for extension，or directly propagate through the natural fractures in its previous direction，and form a complex hydraulic fracture or a fracture network. Dense natural fractures with larger trace length can result in larger fractal dimension of hydraulic fractures and better reformation effect of the fractured reservoirs.