Plants summer-green. Rhizome erect, short, together with basal stipe densely scaly; scales light brown, ovate or ovate-lanceolate, subentire, acuminate. Fronds 20-35 cm; stipe stramineous, 5-12 cm, ca. 2 mm in diam., distal stipe scales sparser toward rachis apex, linear-lanceolate. Lamina bipinnatipartite, light green when dry, elliptic-lanceolate, 20-35 × ca. 5 cm at middle, below middle gradually contracted toward base, acute or shortly acuminate; rachis stramineous, without proliferous bulbils, sparsely scaly abaxially; scales similar to distal stipe scales, light brown, linear-lanceolate. Pinnae 25-35 pairs, attached at nearly right angles to rachis or angled slightly acroscopically, alternate, approximate or slightly approximate, shortly stalked, deltoid-lanceolate, middle pinnae 1.5-2 cm × 6-8 mm at base, bases asymmetrical, acroscopic base auriculate, basiscopic margins truncate, pinnatifid to pinnatisect, apices acute. Lobes 6-8 pairs, angled upward; basal acroscopic auriculate lobes largest, elliptic, pinnatilobate; all remaining acroscopic lobes elliptic, pinnatilobate, forked or entire, apices acute and shortly spinulose; basiscopic lobes entire, apex acuminate and shortly spinulose; both surfaces glabrous; microscales absent, but if present then microscales brown, long acicular; frond texture thinly leathery; venation pinnate, slightly visible abaxially, indistinct adaxially, veinlets simple or forked. Sori terminal on shorter veinlets, 1 per lobe, but basal basiscopic 2 lobes sterile, 2 or 3 on auricles, close to midrib of pinnae, whole lamina fertile; indusia present, large, membranous, with shallow rounded teeth or erose, centers slightly concave, brown when mature.

Polystichum longidens is similar to P. thomsonii, but its lamina apex is shortly acuminate and its pinnae are longer, more deeply lobed, and glabrous on both surfaces.

● On rocks on slopes; ca. 4000 m. Xizang (Lhasa).