Lithographic apparatus is widely recognized as an efficient tool in manufacturing Integrated Circuits (ICs) and other micro–nano structures. During the manufacturing process of ICs, the exposure field is scanned by a narrower illumination field. Illumination uniformity is a key factor in determining resolution and Critical Dimension Uniformity (CDU), which are important performance parameters in advanced lithography systems. To obtain higher photolithography resolution and better CDU, the exposure dose must be kept as uniform as possible in the cross-scanning direction. Improving the consistency of the numerical aperture of each field of relay lens is the premise of ensuring the illumination uniformity. In the optimization conventional imaging optical design, wavefront error, dispersed spot, or optical transfer functions are generally employed as evaluation functions. Therefore, it is impossible to fully and completely satisfying the performance evaluation requirements of the relay lens group using the traditional aberration evaluation method. The general illuminance calculation of the optical system is based on the Monte Carlo method. The computational accuracy degree of the result is related to the number of traces light. In the mainstream optical design software, it is used in a calculation method of four-square cosine of the field angle, in a large number of engineering practice, the difference between the expected illumination distribution of the algorithm and the actual distribution are large. Especially for the telecentric optical system, the result of this calculation method is very worth to be suspected. The relay lens group is used to image the illumination field on the scanning slit plane on the mask plane. The relay lens group has a feature of double telecentric and adjustable pupil, so the above algorithm is still not suitable for the relative illuminance calculation of the image plane of the relay lens group. In this paper, a fast algorithm for calculating the uniformity of illumination is proposed, in which the numerical aperture of the system is calculated by approximate algorithm, and the illumination is characterized by the numerical aperture of exit pupil. The non-uniformity of light field is calculated by this algorithm, which is used as the evaluation function in the process of automatic optimization to optimize the design of relay lens of lithography illumination system. Then the designed relay lens is simulated by the software Light tools. The simulation results show that the non-uniformity of the illumination on the mask surface is less than 0.5% under different coherence factors. Simulation shows that the algorithm results have high correlation with the actual performance, reflecting the actual illumination uniformity, and can improve the uniformity of the system by controlling the value of the evaluation function. The result of the algorithm is proved to have a conservative characteristic, that is, the actual illumination uniformity will exceed the design value, which ensures that the obtained results meet the performance requirements. And this evaluation algorithm has a huge advantage in the speed, which can meet the needs of optimized design. Finally, the integral uniformity of the designed relay lens is tested through experiment, and the results shown that the non-uniformity of illumination is less than 1.21%, which can meet the requirement of the illumination non-uniformity on the mask surface (< 1.5%). It is proved that the fast evaluation algorithm is effective in the optimal design of relay lens.