Due to the introduction of oxygen that works to keep the shape of the fibers and enhance the ceramic yield during high temperature pyrolysis, air-curing is a critical step during the preparation of the polymer-derived Si-Al-C-O fibers (KD-A). In this work, to investigate the evolvement of oxygen in the fibers and the influence of oxygen on the mechanical properties of the resulted KD-A fibers, FT-IR, TGA, XRD, SEM and element analysis were performed on the air-curing process of polyaluminocarbosilane (PACS) fibers and the pyrolysis process of the cured PACS fibers. The results showed that the oxygen in the cured PACS fibers was originated from aluminum aletylacetanate (Al(AcAc)3) and the air-curing process, which could be regarded as a constant mass during the pyrolysis. In addition, it was found that the mechanical properties of the achieved KD-A fibers were greatly effected by the amount of oxygen in the fibers due to its inhabiting the crystallization of silicon carbide. And the oxygen content which leaded to the KD-A fibers with the highest tensile strength was found to be 8~10%.