Creating dwarfmutant germplasm in Panicum miliaceuml and developing Panicum miliaceuml breeding isof great theoretical significance and valuable on application. A character-stably-heritable dwarfmutant in Panicum miliaceuml was induced from Shi hu qian jin mi (planting number 260) by EMS (ethyl methyl sulfonic acid) mutagenesis. The mutants’ agronomic traits, physiological characteristics and dwarfing mechanism were studied from the perspective of morphology, cytology and molecular biology. The results showed that :The mutant was dwarfed at the seedling stage and it would maintain until maturity, the plant height was only 57.1cm. Compared with the wild type,the mutantinternodes were extremely significantly shortened, internodes number has insignificantdifference, tiller number increased significantlyreaching extremely significant level, the drop of pedicel length, ear length and thousand-grain weight reached extremely significant level, no obvious changes appeared on the stem diameter and leaf length, the increase of leaf widthreached extremely significant level and the mutant leaf greened darkly. The chlorophyll contents in sword leaf and top second leaf at heading stage and the sword leaf at mature stage were higher than these in wild type, of whichthe chlorophyll content difference in top second leaf reached significant level compared with wild type. According to the cytological observation of the main stem profile, it was found that the main cause of dwarfing was the decrease of the number of longitudinal cells in the stem node. The plant height frequency of F2 segregation population constructed by the mutant and the high straw J12 distributed unimodally, indicating that the dwarf mutant 819 belonged to quantitative trait inheritance controlled by multiple genes. The endogenous content of GA₃₊₁ in mutant leaves at maturity stage was significantly higher compared with wild type, and the mutant was insensitive to exogenous GA3, indicating thatthe mutant was a GA signal transduction defect type dwarf mutant. A gibberellic acid insensitive gene (accession number: PM01G30960) was identified by BSA-Seq method, which was assumed to be the key gene for the formation of dwarf mutant 819, which laid a foundation for the development and application of new dwarf varieties in Panicum miliaceuml.