Dwarf has widely been introduced in maize breeding to foster the ideal plant type and improve lodging resistance and yield. K123d was derived from the inbred line K123 via spontaneous. The main agronomic traits sensitivity to the gibberellins of dwarf mutant K123d and its wild type K123 were compared in this study. F1, BC, F2 obtained by using a maize dwarf mutant K123d as donor parents and three inbred lines K123, 626, K169 as receptor parents were used in this study to analyze the genetic model of plant height of K123d. K169/K123dF2 was conducted as mapping populations and SSR primers were used to locate the dwarf gene d123 in K123d with BSA. Primers designed from sequence information of br-2 were used to the homology-based cloning of d123. The results showed that: Compared with the Wild type, K123d was shorter stalk by 35.59%, lower ear location, shorter internodes, erecter leaves and poor fertility. The dwarf characteristics of K123d was weak sensitivity to gibberellins. In F2 and BC1 populations the segregation ratio of dwarf to normal plants fit the ratio of 3:1 and 1:1 as expected by χ2-test, the dwarf trait was controlled by one recessive gene. The dwarf gene d123 was mapped on the short arm of chromosome one by SSR markers to a region flanked umc1278 and bnlg1564 with distances of 12.8 cM and 7.3 cM, respectively. The physical distance between markers was 4.2Mbp. The homology-based cloning suggested that d123 be different from br-2 by 12 bases varied. Alignment of the br-2-like domain in d123 with that in br-2 revealed the replacement of glutamate by lysine at the fourth exon. Our results demonstrated that dwarf mutant K123d was a br-2 variable type and it has further research and application value.