Abstract: 6061 aluminum alloy semisolid billet was prepared by the equal-channel angular processing (ECAP)−recrystallization and partial (RAP) process (a combination of equal-channel angular processing and recrystallization and partial remelting). The effects of different process parameters on the alloy microstructure were studied and the quantitative relationship between the process parameters and microstructure was established by response surface methodology (RSM) to optimize the process parameters. According to the orthogonal test, the holding temperature and holding time of the four ECAP−RAP process parameters were found to have the greatest impact on the microstructural characteristics, including average grain size and average shape factor. Through RSM, it was also found that when the average grain size or the average shape factor is optimized separately, another will be degraded. When the two indexes were simultaneously considered, the optimal process parameters were found to be a holding temperature of 623°C and holding time of 13 min, and the corresponding average grain size and average shape factor were 35.97 μm and 0.8535, respectively. Moreover, comparing the experimental and predicted values, the reliability of the established response surface model was verified.