Underground mining causes the redistribution of surrounding rock stress. The value of rock stress is one of the important factors that should be considered when stope structure size is designed. E’xi iron ore body is gently declined in attitude, thin in thickness, and large in occurrence area. According to mining technological conditions and the selected mining method of E’xi iron mine, a typical section of the mine is selected, and it is abstracted into a plane strain problem. In terms of optional stope structure parameters, 9 calculation schemes are planed. Rock stress distributions of stope caused by mining are simulated by the Finite Element Method. Rational stope parameters are chosen by comparing stress distributions of all calculation schemes. Analysis result shows that 8m net distance between pillars, and 4m pillar width are the optimum stope parameters, which not only stabilize the stope but also improve mining efficiency. Choosing stope parameters based on rock stress distributions calculated with numerical simulations, is provided a scientific analysis method for determining rational mining parameters of underground mines.