Herbs, perennial, 3-30(-40) cm tall, often scapiform; rhizome long, slender, sometimes stoloniferous. Stems erect, simple, white (sometimes purple) villous, more densely so upward (sometimes also basally), sparsely to densely minutely stipitate glandular. Leaves basal and cauline; cauline leaves reduced upward, thin, sparsely to moderately villous, sometimes glabrate, sparsely to moderately minutely stipitate glandular, margin entire or remotely serrulate (basal), ciliate, finely 3-veined; basal leaves present, sometimes withered or absent at anthesis, short or long (petiole to 7 cm) petiolate; blade spatulate to oblong to oblanceolate, 1-9.5 × 0.5-1.6 cm, base attenuate, apex rounded or obtuse to acute; cauline leaves usually sessile, oblong or oblong-lanceolate, 1.5-9 × 0.2-1.5 cm, base attenuate to cuneate or ± rounded, usually subclasping; upper cauline leaves lanceolate to lanceolate-linear. Capitula terminal, solitary, 3.5-5(-7) cm in diam.; peduncles sparsely to densely villous, sometimes more densely so below capitula, sparsely to densely glandular. Involucres hemispheric, (1.2-)1.5-2(-3) cm in diam.; phyllaries 2-seriate, subequal, lanceolate, 7-13 × 1-2 mm, abaxially basally densely white lanate to sparsely white villous, usually less so upward, sparsely to densely minutely stipitate glandular, margin scarious, sometimes purple tinged, ciliate, membranous, multiveined, apex acuminate, tip sometimes spreading, often densely villous. Ray florets 31-67, blue or mauve, lavender-blue, rarely pink or pinkish white, lamina linear-lanceolate, 9-25 × 1-2.5 mm, abaxially proximally hairy (crosswalls purple) or glabrous, eglandular; disk florets orange to yellow, 4.5-7 mm, limb funnelform, 3.5-5 mm, proximally sparsely hairy, lobes erect, triangular, 0.7-1.5 mm. Achenes brownish, narrowly obovoid, compressed, 2.5-3.5 mm, sparsely strigillose, sparsely to moderately (apically) minutely stipitate glandular, rarely glabrous and eglandular, margin strigillose-ciliate, 2-ribbed, ribs pronounced. Pappus 4-seriate, whitish (sometimes purplish), bristles barbellate; outermost series of narrow scales 0.6-1.5 mm; outer bristles few, slender, 3.5-5 mm; inner bristles 6-6.5 mm, acute; innermost bristles 6.5-7.5 mm, weakly clavate. Fl. and fr. Jun-Nov. 2n = 54.

Aster flaccidus is used medicinally to treat pulmonary abscesses, tuberculosis, and pertussis.

Three forms have been recognized in China: Aster flaccidus f. flaccidus with leaves ± villous, stem apex and involucre base more densely white villous and inconspicuously glandular, involucres 1.5-2(-3) cm in diam.; A. flaccidus f. glabratus Y. Ling (Fl. Reipubl. Popularis Sin. 74: 239. 1985; NE Xizang) with leaf surfaces glabrous or sparsely villous on abaxial veins only, margin ciliate or not, involucres 2-2.5 cm in diam., phyllaries to 12 mm, abaxially or on veins villous, eglandular; and A. flaccidus f. griseobarbatus Grierson (Notes Roy. Bot. Gard. Edinburgh 26: 131. 1964; 3200-4900 m; S and E Xizang, NW Yunnan [Bhutan, NE India]) with upper stems and involucres purplish brown or gray-brown hairy.

Damp alpine grasslands, alpine and subalpine pastures, meadows, thickets, screes, fallow fields, forests; 1800-5100 m. S Gansu, N Hebei, E Qinghai, S Shaanxi, Shanxi, W Sichuan, Xinjiang, Xizang, NW Yunnan [Afghanistan, Bhutan, India, Kashmir, Kazakhstan, Mongolia, Nepal, Pakistan, Russia (E Siberia), Uzbekistan; SW Asia (Iran)].