Austrocylindropuntia subulata can reach heights up to 4 metres (13 ft), with numerous branches. The elongated, slightly brittle branches are up to 50 centimetres (20 in) long. The stems are marked by rhomboid to ovate bumps in a few spiral rows. At the tip of each bump is the areole, from which grow one to four straight, grayish-white spines up to 8 centimetres (3 in) long. The awl-like rudimentary leaves are up to 12 centimetres (5 in) long. The red flowers are up to 6 centimetres (2+1⁄4 in) long. The long, warty hypanthium is covered by rudimentary leaves up to 2 centimetres (3⁄4 in) long. The fruit are ovoid or club-like and sometimes thorny. They are up to 10 centimetres long.