Freeform pupil illumination technology is an important photolithographic resolution enhancement technology for immersion photolithography machines locating in nodes at 28 nm and below. Arbitrary illumination mode could be realized by adjusting the angular spectrum of the beam by micro mirror array (MMA). The angular position distribution of MMA is of great significance to the application of the freeform pupil illumination technology. An angular position distribution algorithm of MMA based on genetic algorithm is proposed in this paper. Compared with the angular position distribution algorithm of MMA based on the simulated annealing algorithm, the iteration speed of the proposed algorithm is increased by more than 10 times, and the MMA angular position distribution obtained by the proposed algorithm can accurately reproduce the distribution of the target pupil intensity. Results of the photolithography performance simulation show that the root mean square (RMS) values of the asymmetry distribution of the photoresist exposure patterns of the algorithm pupil and the target pupil are basically the same, and the RMS values of critical dimension difference distribution are less than 0.5 nm.