  Based on first-principles calculations and tight-binding model analysis, we
propose that black phosphorus (BP) can host a three-dimensional topological
node-line semimetal state under pressure when spin-orbit coupling (SOC) is
ignored. A closed topological node line exists in the first Brillouin zone (BZ)
near the Fermi energy, which is protected by the coexistence of time-reversal
and spatial inversion symmetry with band inversion driven by pressure.
Drumhead-like surface states have been obtained on the beard (100) surface. Due
to the weak intrinsic SOC of phosphorus atom, a band gap less than 10 meV is
opened along the node line in the presence of SOC and the surface states are
almost unaffected by SOC.
