Machaerina angustifolia is a perennial plant with short rhizomes. Its culms are biconvex, ranging from 30 to 130 cm in height and 4 to 8 mm in width, characterized by their smooth surface. The leaves are primarily basal or nearly so, equitant, and typically as long as or slightly longer than the culm, with widths between 6 and 23 mm, featuring a subacute apex and smooth, pale green surfaces. The leaf margins are entire, and the sheaths are brown. The inflorescence is a large decompound panicle, 30 to 50 cm long and 8 to 16 cm wide, consisting of 4 to 5 partial panicles with branches exserted from the bract sheath, with congested spikelets in clusters of 2 to 4, each turbinate in fruit, measuring 7 to 8 mm long. The spikelets contain up to three flowers, surrounded by 5 to 8 dark brown, subcoriaceous, oblong-ovate glumes, which are boat-shaped with a conspicuous keel. The flowers can be either bisexual (perfect) or staminate, with three stamens and a superior ovary with a three-cleft, pubescent style. The calyx (sepals) and the corolla (petals) are absent. The fruits are yellowish-brown achenes, narrowly ellipsoid, trigonous, slightly winged, and 2.8 to 3.2 mm long, including a three-winged stipe approximately 1.5 mm long. The seeds are one per achene, with a thin seed coat free from the pericarp. They have a basal embryo with an abundant endosperm. It inhabits wet forests and bog margins between 420-2070 meters.