Prism beam expander (PBE) is widely used in the spectral narrowing module of excimer lasers. The beam divergence and fluence could be reduced by the prism beam expander. The prism number, magnification of single prism and apex angle of the PBE should be optimized to improve the efficiency of the laser. Based on the magnification theory of the prism, the impact of the incidence angle, apex angle and exit angle of the prism on magnification are analyzed by numerical simulation. The dependence of magnification on incidence angle is demonstrated experimentally. Furthermore, to achieve a given linewidth of the laser, the minimal magnification of the line narrowing module is deduced. In addition, CaF2 prism beam expander with magnification of 13.3 is designed. Finally, 0.915 pm linewidth ArF laser is obtained using the designed PBE. The experimental results agree with the theoretical analysis.