Abstract: During multi-stage fracturing in unconventional reservoirs, induced stress is the key factor in natural fracture opening and fracturing effets.The in-situ stress distribution analytical model was derived, and the mechanical conditions under which natural fractures initiated due to tensile failure and shear failure were obtained based on rock mechanics analysis.Stress distribution of Well Hei 185 by the model showed that induced stress existed in horizontal well fracturing and it made the in-situ stress along wellbore increased 1.70 MPa and the state of the in-situ stress changed.If induced stress was taken into account, minimum pump pressure required to activate natural fractures was 29.27 MPa rather than 26.31 MPa.The induced stress in multi-stage fracturing made the activation of natural fractures difficult.With the increase of induced stress, pump pressure to activate natural fractures increases, and there is a linear relation between induced stress and pump pressure.The induced stress should be considered into account in hydraulic fracturing design.