Multiferroic properties of BiFeO_3 single-wall nanotube are calculated using the density functional theory. We find that it characteristically exhibits the double-helix distributions of polarization and magnetic moment, while those in BiFeO_3 bulk are uniform. This allows BiFeO_3 single-wall nanotube to have the chirality of double-helix multiferroic properties. Furthermore, torsion of BiFeO_3 single-wall nanotube switches the double-helix chirality.