Polyaluminocarbosilane (PACS) is an important precursor for silicon carbide (SiC) fibers and ceramics. The structure of PACS and the oxidative curing, thermal pyrolysis, and sintering effect of Al have already been substantially studied. However, the structural evolution of polyaluminocarbosilane itself during the polymer–ceramic conversion process, especially the changes in the structure forms of Al, are still pending questions. In this study, PACS with a higher Al content is synthesized and the above questions are elaborately investigated by FTIR, NMR, Raman, XPS, XRD, and TEM analyses. It is found that up to 800–900 °C the amorphous SiOxCy, AlOxSiy, and free carbon phases are initially formed. With increasing temperature, the SiOxCyphase partially separates into SiO2then reacts with free carbon. The AlOxSiyphase changes into Al3C4and Al2O3by reaction with free carbon at around 1100 °C. The complicated reactions between Al3C4, Al2O3, and free carbon occur, leading to the formation of the Al4O4C and Al2OC phases at around 1600 °C, which then react with the SiC and free carbon, resulting in the formation of the Al4SiC4phase at 1800 °C. The amorphous carbon phase grows with the increasing temperature, which then turns into a crystalline graphitic structure at around 1600 °C. The growth of β-SiC is inhibited by the existence of the Al4O4C, Al2OC, and Al4SiC4phases, which also favor the formation of α-SiC at 1600–1800 °C.