Plant height reduction is pivotal for improvement in lodging resistance in wheat and therefore has great impacts on high yield stability. Reduced Height 8 (Rht8) gene has been widely used in wheat breeding for several decades and is well adapted to warm and dry environment; however, its identity remains unknown due to the complexity in wheat genome. We have identified two new Rht8 allelic mutants named Rht8-2 and Rht8-3 in the wild type background of Jing 411 (WT) from our in-house wheat mutant library, and these mutants showed 20% of height reduction and had no obvious negative impact on grain yield. By using two large independent mapping populations, we fine mapped Rht8-2 and Rht8-3 to a genomic region of ~700 kb and 170 kb on chromosome 2DS, respectively. Based on the assembly of Jing 411 genome, we found TraesCSU03G0022100 gene encoding an RNase H-like protein located in the mapped region, and the two mutants possessed distinct mutations that ultimately lead to premature stop codons in TraesCSU03G0022100. We also found that the original Rht8 variety Akakomugi harbored identical mutation as Rht8-2. Gene editing and near-isogenic lines analyses suggest that mutation of this candidate gene lead to a significant height reduction. To investigate the effects of GA on dwarf phenotype of Rht8, we treated the two mutants and WT with exogenous GA3 and found that GA3 could effectively restore the semi-dwarf phenotype of Rht8-2 and Rht8-3. Moreover, GA biosynthetic genes, such as GA20ox-2 and GA13ox, were significantly down-regulated in both mutants. Endogenous GA content analysis showed that the level of bioactive GA3 was significantly decreased while the level of GA4 was significantly increased in the two mutants comparing to that of WT. We also found that dwarfing Rht8 gene originated after hexaploidization and become more and more popular in modern wheat breeding. The characterization of Rht8 paves the way for efficient and precise engineering of dwarfing genes in modern wheat breeding.