Natural fractures are generally well developed in most hydrocarbon and geothermal reservoirs, which can produce complex fracture networks due to the activation of fractures during hydraulic stimulation. The present paper is devoted to developing a method to investigate the activation characteristics of fracture under injection-shearing coupled condition at laboratory scale. The fluid is injected into the single-fractured granite until the fracture is activated based on the triaxial direct shear tests. The results show that injection process can significantly influence the shear stress distribution field, resulting in release of shear stress and relative slip between the opposite sides of the fractured surface. The injection-induced activation of fracture is strongly dependent on the stress states. When the normal stress in-creases, the injection-induced activation pressure increases, and the comparatively high normal stress can restrain the fracture activation. The fracture deformation mechanisms during fluid injection are also discussed preliminarily with the experimental data. The sensitivity of shear stress to fluid injection in-creases with increase of shear stress level, while it decreases under high normal stress. The results can facilitate our understanding of the natural fracture activation behavior during fluid pressure stimulation.