Lachnagrostis filiformis is a clump-forming perennial plant with erect or decumbent culms at the base, ranging from 20 to 60 cm in height and 1 to 2 mm in diameter, with a glabrous surface. It has a fibrous root system. The leaves are simple and alternate, with flat blades 10 to 20 cm long and 0.5 to 3 mm wide, featuring an acuminate apex. The upper surfaces of the leaves are rough, while the lower surfaces are smooth. The leaf margins are entire, and the veins run parallel. The sheaths loosely clasp the stem, with overlapping margins, and can be either glabrous or rough. A noticeable ligule is present, which is membranous and linear-triangular, measuring 6 to 8 mm in length, with entire margins and an acuminate apex that often appears frayed. The flowers are paniculate, terminal inflorescences with numerous flexuous and rough branches that ascend to spread out, giving a diffuse appearance and measuring 10 to 30 cm long. The spikelets (pictured) are numerous and only found on the distal third to half of the primary inflorescence branches. They are pale, with a rachilla partially hidden by silky hairs. The glumes are 2 to 3 mm long and enclose the floret completely, with a prominent, rough to ciliate keel. The lemma is 1.5 to 2 mm long, silky pubescent, with the upper quarter bifid and an awn attached near the middle, measuring 3 to 4 mm long and geniculate. The palea is approximately 1.3 mm long, narrow, glabrous, with an acute apex. It typically has three stamens. The fruit is an ellipsoid, brown caryopsis, roughly 1.5 mm long and 0.5 mm wide, with a smooth surface. It is commonly found in disturbed or open areas between 260-2520 meters.