Abstract: To discover the mechanism that generates complex fracture network in hydraulic fracturing of shale gas reservoirs, a numerical simulation was conducted hydraulic fracturing on shale cracking based on particle flow theory.Shale particle meso-mechanical parameters were retrieved by a parallel PSO algorithm with shale rock mechanical parameters.Brazilian tests and hydraulic fracturing tests with two preset cracks were carried out with the PFC discrete element model, with the next step that the effect of interaction between fractures on propagation was analyzed through varying test parameters.Numerical simulations demonstrated that when the preset crack was 30°or 45°inclined in the Brazilian test, the vertical crack turned into the preset crack and initiated from the sides of the preset crack.At that point, the stress-strain curve turned from elastic deformation to plastic deformation;when the preset crack was inclined 90°, the vertical crack propagated across the preset crack, and stress-strain curve demonstrated mainly elastic deformation.The smaller inclined angle of preset crack in the hydraulic fracturing test, and the easier preset crack shears, the more readily vertical crack changes its propagation direction, and further the more complicated propagation paths.The results demonstrated that the inclined angle of crack exerts significant influence on hydraulic fracture networks in shale gas reservoir;and after rock strain reaches is peak, the axial stress-strain curve decreases and then increases in a cosine wave relation.At that point, fracture networks become more complicated with decreasing inclined angle.