A bushy shrub, Banksia aculeata grows up to 2 m (7 ft) tall, with fissured grey bark on its trunk and branches. Unlike many other banksia species, it does not have a woody base, or lignotuber. The leaves range from 4–9 cm (1 1⁄2–3 1⁄2 in) long and 0.8–3 cm (1⁄4–1 1⁄4 in) wide, with sharply pointed rigid lobes on the margins. Appearing in February and March, the cylindrical flower spikes—known as inflorescences—range from 6–9 cm (2 1⁄4–3 1⁄2 in) long, growing at the ends of short leafy 2–3-year-old side branches. Hanging downward rather than growing upright like those of most other banksias, they are composed of a central woody spike or axis, from which many compact individual flowers arise perpendicularly. These floral units are made up of a smooth tubular perianth that envelops the flower's sexual organs. The perianth is 3.0–4.3 cm (1 1⁄4–1 3⁄4 in) long and pink at the base grading into cream. In late bud, the end of the perianth has a characteristic four-angled (squarish) appearance. It then splits at anthesis to reveal the smooth straight pistil, which is slightly shorter than its enveloping structure at 3.0–4.2 cm (1 1⁄4–1 3⁄4 in) long. The fruiting cone, known as an infructescence, is a swollen woody spike in which up to 20 massive follicles are embedded; the withered flower parts persist on the spike, giving it a hairy appearance. Oval in shape, the follicles are wrinkled in texture and covered with fine hair. They are 3.0–4.5 cm (1 1⁄4–1 3⁄4 in) long, 2.5–3.0 cm (1–1 1⁄4 in) high, and 2.0–2.5 cm (3⁄4–1 in) wide.
The obovate (egg-shaped) seed is 4–5 cm (1 5⁄8–2 in) long and fairly flattened. It is composed of the wedge-shaped seed body (containing the embryonic plant), measuring 1.0–1.2 cm (3⁄8–1⁄2 in) long by 1.5–1.8 cm (5⁄8–3⁄4 in) wide, and a papery wing. One side, termed the outer surface, is grey and wrinkled and the other is black and sparkles slightly. The seeds are separated by a sturdy dark brown seed separator that is roughly the same shape as the seeds with a depression where the seed body sits adjacent to it in the follicle. Known as cotyledons, the first pair of leaves produced by seedlings are cuneate (wedge-shaped) and measure 1.1–1.2 cm (3⁄8–1⁄2 in) long by 1.9 cm (3⁄4 in) wide. They are dull dark green, sometimes with a reddish tinge, and the margin of the wedge is convex. The auricle at the base of the cotyledon leaf is pointed and measures 0.3 cm (1⁄8 in) long. The hypocotyl is thick, smooth and dark red. The obovate to oblong seedling leaves are 4–9 cm (1 1⁄2–3 1⁄2 in) long by 2.0–2.5 cm (3⁄4–1 in) wide with serrated margins, v-shaped sinuses and sharp teeth.
The related Banksia caleyi is similar in appearance but can be distinguished by its recurved (downward curving) leaf margins, and smaller follicles and perianths. Its flowers appear from October to December.