Geostress is an important geomechanical property of underground rock, and an accurate evaluation of its magnitude and direction is important for the stimulation scheme design of tight oil reservoirs. In this study, taking a tight oil sandstone reservoir as an example, a comprehensive evaluation of the magnitude and direction of geostress under the constraints of rock mechanics, differential strain, hydraulic fracturing, and microseismic monitoring was systematically performed. The results showed that the three principal stresses are functions of the burial depth. Fracturing is a tensile fracture that is directly affected by the horizontal minimum principal stress; thus, a good positive correlation exists between the fracture pressure and the horizontal minimum principal stress. No direct relationship exists between the rupture pressure and the maximum horizontal principal stress, which is primarily affected by the rock strength and reflects Poisson’s ratio. Therefore, the maximum horizontal principal stress of rocks with high rupture pressures may be relatively low. Based on the well-wall caving method, drilling-induced fracture method, focal mechanism analysis, and microseismic monitoring; the present crustal stress direction of the target layer was determined to range NE45°~NE60°. The existence of natural fractures leads to a deflection in the expansion direction of local artificial fractures, and the expansion of pressure fractures is primarily affected by the distribution and opening of natural fractures. Furthermore, the hydraulic fracture height and half-fracture length showed a negative correlation, and the natural fracture opening influenced the hydraulic fracture height. This study provides scientific guidance for evaluating fracturing effects in highly heterogeneous tight oil reservoirs.