Plant height is one of the key factors determining yield and stability in maize. This research aims to increase the individual yield of maize by optimizing the plant architecture. We have discovered a dwarf mutant, d309, in the mutagenized material of the maize variety B73, which was stably inherited through continuous selfing. Genetic analysis revealed that the phenotype of the d309 mutant is controlled by a single recessive gene. Compared to the B73 inbred line, the d309 mutant exhibited a significantly reduced plant height and shortened internodes. Cytological studies indicated that the cell length between the 5th and 6th internodes of the d309 mutant was significantly reduced. By constructing an F2 population from the cross of d309 and PH6WC and employing BSR-seq technology, we preliminarily mapped the mutation site to an interval of 3.47-17.47 Mb on chromosome 3. Further gene mapping localized the mutation site between two molecular markers, 5-4 and 5-7, within which there is only one open reading frame encoding gibberellin 3-oxidases (GA3ox, Zm00001d039634), which has been reported as Dwarf1. Sequencing results showed that the d309 mutant has a single nucleotide mutation in the CDS region of the Zm00001d039634 gene, leading to an amino acid sequence change. Protein structure prediction suggested that the mutation site is located within the Fe(II)2OGD (iron 2-oxoglutarate dioxygenase) domain. Exogenous application of gibberellin restored the dwarf phenotype of the d309 mutant, confirming it as a gibberellin synthesis-deficient mutant. Additionally, the d309 mutant differs from the reported Dwarf1 allelic mutants in its mutation mechanism, indicating that d309 is a new allelic variant of the Dwarf1 gene.