Miscanthus (Geodae-Uksae in Korean, GU) is a promising source of biomass for biosugar production due to its superior mass yield and minimal need for nitrogen fertilizer and herbicides. In this study, the biomass productivity, chemical composition, and physical properties of Miscanthus were investigated for two harvest dates. The total biomass of early harvest GU (EH-GU; 20.1 t DM/ha) was greater than that of the delayed-harvest GU (DH-GU; 18.0 t DM/ha). There was no noticeable difference in the carbohydrate content between the EH-GU and DH-GU, while the lignin content remarkable decreased after pretreatment. The enzymatic conversion rates of GU to biosugar increased after pretreatment, to approximately 86 to 90%. After enzymatic hydrolysis, 381 g and 147 g of glucose and xylose were produced, respectively. Consequently, 528 g of biosugar was obtained from 1 kg of EH-GU. Determining the optimal harvest date of Miscanthus has enabled a more efficient enzymatic hydrolysis and higher biosugar yields.