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Considering that the presence of a leafy head is a key morphological characteristic that determines the yield and quality of Chinese cabbage, identifying the major genes regulating the formation of a leafy head is crucial for variety improvement. A heading-related gene, <i>BrKS</i>, was previously predicted from a non-heading mutant, <i>nhm1</i>, derived from a heading variety, which encodes a key enzyme involved in gibberellin biosynthesis. Here, another mutant, <i>nhm2</i>, was identified from the same EMS-mutagenized population, and the phenotype of <i>nhm2</i> was consistent with that of <i>nhm1</i>. We crossed mutants <i>nhm1</i> and <i>nhm2</i>, and their F₁ plants exhibited the mutant phenotype, which indicated that their mutant genes were allelic. A single non-synonymous mutation in the fourth exon of <i>BrKS</i> in mutant <i>nhm1</i> and another single non-synonymous mutation in the tenth exon of <i>BrKS</i> in mutant <i>nhm2</i>. The same gene mutation in mutants <i>nhm1</i> and <i>nhm2</i> produced a similar non-heading phenotype, which confirmed the role of <i>BrKS</i> in the leafy head formation of Chinese cabbage. RNA-Seq analysis indicated that a transcription factor gene, <i>BrERF1A</i>, which is associated with leaf development, significantly down-regulated expression in mutant <i>nhm1</i>, and after the mutant was treated with GA₃, the expression level of <i>BrERF1A</i> was recovered, which indicated that <i>BrKS</i> might be involved in leafy head formation through regulating the expression level of <i>BrERF1A</i>. Our findings provide important clues for revealing the molecular mechanism of leafy head formation in Chinese cabbage.