Using a cytoplasmic male sterile （CMS） line of Brassica napus carrying cytoplasm from Brassica oxyrrhina as the maternal parent， we developed a stable CMS line in flowering Chinese cabbage （Brassica rapa subsp. parachinensis） through interspecific hybridization followed by six backcrosses， assisted by embryo rescue and a speed breeding system. The line was comprehensively characterized for morphological traits， pollen fertility， seed-setting performance， and cytoplasm-specific molecular markers. The CMS line exhibited typical floral morphology of flowering Chinese cabbage， featuring degenerated stamens with shortened filaments and shriveled anthers while nectaries developed normally. Acetic acid-carmine staining revealed aborted pollen grains， characterized by small， round， anucleate， and stained pale brown. Scanning electron microscopy further showed a lack of sporopollenin deposition and the collapse of germination apertures in the sterile pollen， consistent with classical CMS phenotypes. Field evaluations confirmed complete self-sterility； however， under open pollination， plants produced an average of 12 seeds per silique， indicating strong outcrossing compatibility. Comparative mitochondrial genome analysis enabled the development of a cytoplasm-specific KASP marker， KOX3， which reliably discriminates cytoplasm from Brassica oxyrrhina （C/C genotype） from other cytoplasmic types （T/T genotype）. This novel CMS line provides a valuable germplasm resource for heterosis breeding in flowering Chinese cabbage， and the KOX3 marker offers a robust molecular tool for rapid cytoplasm identification and purity assessment of CMS lines.