A novel trapezoid prism homogenizer is used to improve the uniformity of output energy density of excimer laser. The principle and the design requirements of the novel trapezoid prism are analyzed theoretically, and the optimal position of uniform section is calculated. Using this novel homogenizer, the fluctuation of beam uniformity of XeCl excimer laser is less than 4%. The result is better than that obtained from a normal prism homogenizer. The evaluating norms of excimer laser beam uniformity are analyzed in detail. The process window defines uniformity requirements of excimer laser, while the energy fraction answers the problem of useful percentage of energy within a giving process window. The top-hat factor defines the uniformity in the whole energy range, and the dynamic range is the variation range of screen-homogenizer in the specific application. Using this evaluating norm the uniform beam obtained by the novel trapezoid prism homogenizer is analyzed.