A method for optimizing a computer-generated hologram (CGH) for high-stability laser processing is proposed. The CGH is optimized during laser processing; therefore, unpredicted dynamic changes in the laser processing system, in addition to its static imperfections, are automatically compensated for by exploiting the rewritable capability of the spatial light modulator. Consequently, the short-term and long-term stability are improved, which will contribute to the realization of high-speed, high-precision laser processing. A CGH that generated 36 parallel beams was continuously optimized, and the maximum uniformity reached 0.98, which is higher than reported in previous research. To the best of our knowledge, this is the first demonstration of gradual improvement of parallel laser processing with in-process optimization of the CGH. Furthermore, it was also demonstrated that the performance of the laser processing system against unexpected disturbances was improved.