Rice is an important food crop in the world. The awn may protect rice seeds from being cut by birds, which is important in rice domestication, survival and diffusion. However, the characteristic of awn is gradually washed out during rice domestication and artificial selection. Mapping and cloning of rice awn genes is the basis of studying the genetic mechanism of awn domestication. In this study, 146 chromosome segment substitution lines (CSSLs) derived from DongNanHui 810/ZhangPu wild rice with DongNanHui 810 as the recurrent parent were used to analyze the quantitative trait loci (QTL) controlling the long awn of rice. The results showed that four CSSLs contained one QTL for the long awn. Using substitution mapping, the GAD1-2 gene was mapped between two markers (Ind8-10 and RM4936) on chromosome 8, with a genetic distance of about 4.75 Mb. Using the dominant individuals of segregating populations, the GAD1-2 gene was eventually located between two Indel markers, with a physical distance of about 27 kb, which contained only two candidate genes Os08g0485500 and Os08g0485400. Sequencing analysis showed that Os08g0485500 was the candidate gene of GAD1-2. Further analysis showed that there were six bases missing in the conservative ORF region, resulting in the absence of serine and cysteine that led to the long awn of the four CSSLs. The GAD1 gene was also cloned in this position, suggesting that GAD1-2 and GAD1 were allelic. This study laid a foundation for further understanding of the genetic regulation mechanism and genetic evolution of the awn gene in rice.