Hot　dry　rock　(HDR)　is　deep　geothermal　reservoir　rock　predominantly　containing　granite,　whose　physical　and　mechanical　properties　are　governed　by　heterogeneous　crystal　structures.　However,　the　fluid　conductivity　of　intact　granite　with　ultra-low　permeability　cannot　be　reformed　using　existing　techniques.　Complex　fracture　networks　must　be　created　by　connecting　hydraulic　fractures　with　natural　fractures　developed　in　reservoirs.　In　this　study,　we　introduce　the　wall　barrier　method　into　the　novel　hydro-grain-based　model　(hydro-GBM)　to　build　a　prefractured　granite　model　at　the　mineral　grain　scale.　This　model　enables　investigation　of　the　effects　of　the　stress　environment,　mineral　spatial　distribution,　and　fluid　injection　rate　on　hydraulic　fracturing　characteristics.　Microcracks　around　the　inner　tips　of　pre-existing　fractures　reproduce　the　initiation,　propagation,　and　coalescence　behaviors　similar　to　experimental　results.　Mineral　spatial　distributions　induce　random　changes　in　the　propagation　paths　of　hydraulic　fractures.　Increasing　the　fluid　injection　rate　delays　the　deflection　of　hydraulic　fractures　and　extends　the　propagation　distance　along　the　major　axes　of　pre-existing　fractures.　The　average　activity　level　and　proportion　of　large　seismic　events　are　reduced　by　injecting　low-rate　fluid.　In　summary,　our　results　reveal　the　interactions　between　hydraulic　and　pre-existing　fractures　and　provide　a　valuable　reference　for　constructing　an　efficient　enhanced　geothermal　system　(EGS)　for　deep　reservoirs.