The total flavonoid contents in four common energy grasses (hybridPennisetum, switchgrass, giant reed, elephant grass) including whole strain, stems and leaves were quantified. The flavonoids were detected by LC-MS. For the whole strains, switchgrass has a flavonoids content of 6.84 mg quercetin equivalents/g dry weight, which is 59.8%, 20.1% and 11.2% higher than giant reed, hybridPennisetum and elephant grass. The content in leaves is 2.3 ~ 3.0 times to stems, the contents in hybridPennisetum leaves is 10.55 mg quercetin equivalents/g dry weight, which is the highest among all these samples. 14 flavonoid glycosides and 2 flavonols were identified in the extracts of four grasses. Quercetin, isorhamnetin and quercitrin are identified positively with standards. This research may provide a theoretical support for the value added phytochemicals extract from energy grass.