Forage sorghum (Sorghum bicolor (L.) Moench) is a C4 cereal crop with excellent quality, which is widely cultivated in many countries and regions. Acid detergent fiber is widely found in the stem and leaf tissues of plant, which is the main substance affecting the digestibility of livestock. It has a very important impact on the milk yield and quality of cows, dairy sheep and pigs. In order to further understand its genetic mechanism, we re-sequenced 206 forage sorghum germplasms from different regions of the world and identified 14,570,430 SNPs and 1,967,033 indels. Based on SNP markers, we analyzed the population genetic structure and identified the gene loci related to acid detergent fiber content by genome-wide association analysis (GWAS). Genetic relationship between materials showed that Asian and American sorghum varieties, breeding lines and improved varieties were more diverse, while European varieties were relatively more closely related. These findings provide new clues and directions for further study. GWAS revealed that 8 QTLs harboring 91 genes were found to be associated with acid detergent fiber content. These genes were significantly enriched into 6 major genes involved in cell membrane material transport or enzymes, showing a regional distribution. The findings provide a basis for us to understand the origin and spread of haplotypes related to acid detergent fiber content. The findings will accelerate the study of genetic gain of acid detergent fiber content and help breeders to improve feed quality and stress tolerance of forage sorghum.