Abstract: Polyaluminocarbosilane (PACS) is a precursor of ultra-high-temperature-resistance Si-Al-C fiber. In order to get a desirable precursor of the fiber, polysilacarbosilane (PSCS) was used to react with aluminum acetylacetonate (Al(acac)₃) to get PACS, the reaction process was traced by FTIR, GPC, ¹H-NMR, ²⁷Al-NMR and UV-Vis spectra, and the production mechanism was studied in detail. The result shows that PSCS is a kind of oligomers with Si-Si-Si and Si-C-Si bonds. There are two kinds of reactions during the whole process. One is Kumada rearrangement that Si-Si-Si bond translated into Si-C-Si bond, the other is Al(acac)₃ with the silicon carbon compounds. The obtained PACS has a higher relative molecular mass and branching degree because of the cross linkage of Al(acac)₃. The crosslink mainly occurs below 330℃ and above 400℃. In PACS, Al is in the state of Si-O-Al.