Black　phosphorus　(BP)　has　been　enjoying　popularity　in　mechanical　and　optoelectronic　devices　due　to　its　superior　anisotropic　puckered　structure.　However,　the　vibrational　properties　of　BP　remain　unexplored　which　greatly　limits　its　wide　applications　in　resonance-based　nano　electromechanical　system　(NEMS)　devices.　Therefore,　in　order　to　design　mechanically　reliable　ultra-sensitive　nanosensors,　molecular　dynamics　simulations　are　utilized　to　explore　the　vibrational　properties　of　monolayer　BP.　We　have　systematically　assessed　the　effects　of　the　initial　actuation　amplitude,　internal　vacancy　density　and　mechanical　strain　on　the　vibrational　properties　of　monolayer　BP　along　the　armchair　and　zigzag　directions.　It　is　found　that　the　pristine　monolayer　BP　exhibits　lower　energy　disspation　and　smaller　dominant　resonant　frequency　during　free　vibration　along　the　armchair　direction　than　along　the　zigzag　direction,　indicating　the　anisotropic　mechanical　vibrational　properties　of　monolayer　BP,　which　further　result　in　anisotropic　multimode　resonant　characteristics　in　the　vibration　frequency　spectrum.　Additionally,　the　dominant　frequency　monotonically　decreases　gradually　with　increment　of　the　internal　vacancy　density,　while　increasing　with　increment　of　the　initial　actuation　amplitude　and　the　mechanical　strain.　The　energy　dissipation　increases　gradually　with　increment　of　the　initial　actuation　amplitude　and　internal　vacancy　density,　but　decreases　under　mechanical　strain.　Overall,　these　findings　provide　a　fundamental　understanding　of　the　resonant　properties　of　monolayer　anisotropic　BP　that　sheds　light　on　its　applications　in　micro-or　nano-resonator-based　devices.　©　2020　IOP　Publishing　Ltd.