Abstract: A method for determining grid cell size in geomechanical modeling of fractured reservoirs including a variation range of mechanical parameters of the reservoir is determined. A three-dimensional fracture discrete network model is established. Mechanical parameters of fracture surface are determined on the basis of fracture surface mechanical test. Equivalent mechanical parameters of models with different sizes are researched by three-cycle method, and size effect and the anisotropy of the mechanical parameters of the fractured reservoir are calculated respectively, and an optimal grid cell size in geomechanical modeling is determined.