Taking into account the theoretical resolution of the concave gratings, the aberration, the width and height of the slit and the size of the grating, mathematical expression for the resolution of the system was deduced and a new method for simulating and testing the system was proposed. We drew some following conclusions. The system can not improve the resolution by increasing indefinitely grating lines. The energy and the resolution are considered to select an appropriate width of the slit. The impact of the width of the slit on the system resolution is far greater than that of the height of the slit and the theoretical resolution of the concave grating. And then, a formula of the size of the gratings and a method for optimizing were proposed. The theoretical calculation values coincide with the experimental results.