Chlorophyll (Chl) is an essential component of the photosynthetic apparatus and pigments in plant greening. Leaf color is an important agronomic and commercial trait of Chinese cabbage. In this study, we identified a pale green mutant pgm created by ethyl methane sulfonate (EMS) mutagenesis in Chinese cabbage. Compared with wild-type (FT), pgm had a lower Chl content with a higher Chl a/b ratio, imperfect chloroplast structure, and lower non-photochemical quenching. However, its net photosynthetic rate and biomass showed no significant differences. Genetic analysis revealed that the pale green phenotype of pgm was controlled by a recessive nuclear gene, designated as Brpgm. We applied BSR-Seq, linkage analysis, and whole-genome resequencing to map Brpgm and predicted that the target gene was BraA10g007770.3C (BrCAO), which encodes chlorophyllide a oxygenase (CAO). Brcao sequencing results showed that the last nucleotide of its first intron changed from G to A, causing the deletion of the first nucleotide in its second CDS and termination of the protein translation. The expression of BrCAO in pgm was upregulated, and the enzyme activity of CAO in pgm was significantly decreased. These results provide an approach to explore the function of BrCAO and create a pale green variation in Chinese cabbage.