The St genome of Pseudoroegneria (Triticeae, Poaceae) not only accounts for more than 60% perennial speciation, but also prominent for forage and crop breeding. The diploid Pseudoroegneria libanotica with more ancient St genome is covered by cuticular wax on the aerial part, and exhibited strong drought resistance. To reveal the genetic relationship among Triticeae species and illustrate the mechanism between water deficit and cuticular wax formation, in this study, we (1) assembled the chromosome level St genome of Pse. libanotica with 2.99 Gb assembled into seven pseudochromosomes; less repeat clusters (TEs) is the main reason for the St genome with smaller genome size, and high genome heterozygosity might cause abundant St-containing speciation; (2) the genus Pseudoroegneria diverged during the middle and late Miocene, and unique genes, gene family expansion and contraction in Pse. libanotica were enriched in biotic and abiotic stresses, such as fatty acid biosynthesis which may be greatly contribute to the its drought adaption; (3) in total, 14 genes were involved in wax biosynthesis under 28 days drought treatment, more importantly, a new Kcs gene evm.TU.CTG175.54 plays a critical role in the very long chain fatty acid (VLCFA) elongation from C18 to C26 in Pse. libanotica. Our study lays a foundation for the genome diversification of Triticeae species and deciphers cuticular wax formation genes that have contributed to the drought resistance of Pse. libanotica.