Discrete fracture network (DFN) commonly existing in natural rock masses plays an important role in geological complexity which can influence rock fracturing behaviour during fluid injection. This paper simulated the hydraulic fracturing process in lab-scale coal samples with DFNs and the induced seismic activities by the discrete element method (DEM). The effects of DFNs on hydraulic fracturing, induced seismicity and elastic property changes have been concluded. Denser DFNs can comprehensively decrease the peak injection pressure and injection duration. The proportion of strong seismic events increases first and then decreases with increasing DFN density. In addition, the relative modulus of the rock mass is derived innovatively from breakdown pressure, breakdown fracture length and the related initiation time. Increasing DFN densities among large (35–60 degrees) and small (0–30 degrees) fracture dip angles show opposite evolution trends in relative modulus. The transitional point (dip angle) for the opposite trends is also proportionally affected by the friction angle of the rock mass. The modelling results have much practical meaning to infer the density and geometry of pre-existing fractures and the elastic property of rock mass in the field, simply based on the hydraulic fracturing and induced seismicity monitoring data.