TmFeO3　exhibits　rich　physical　properties　such　as　magneto-optical　effect,　multiferroicity,　and　spin　reorientation,　making　it　possess　significant　research　value　in　condensed　matter　physics　and　materials　science.　In　this　study,　we　utilize　a　time-domain　terahertz　magneto-optical　spectroscopy　system　to　investigate　the　changes　in　spin　resonance　frequency　of　TmFeO3　single　crystal　at　T　=　1.6　K　under　external　magnetic　fields　in　a　range　of　0–7　T.　The　TmFeO3　sample　is　grown　in　an　optical　floating　zone　furnace　and　its　crystallographic　orientation　is　determined　by　using　back-reflection　Laue　X-ray　photography　with　a　tungsten　target.　The　measurement　setup　is　a　self-built　time-domain　terahertz　magneto-optical　spectroscopy　system,　with　magnetic　fields　in　a　range　of　0–7　T,　temperatures　in　a　range　of　1.6–300　K,　and　a　spectral　range　of　0.2–2.0　THz.　A　pair　of　1　mm-thick　ZnTe　nonlinear　crystals　is　used　to　generate　and　detect　terahertz　signals　through　optical　rectification　and　electro-optic　sampling　technique.　The　system　variable　temperature　and　magnetic　field　are　controlled　by　a　superconducting　magnet.　In　experiments,　a　linearly　polarized　terahertz　wave　is　vertically　incident　on　the　sample　surface,　and　its　magnetic　component　HTHz　is　parallel　to　the　sample　surface.　By　rotating　the　sample,　the　angle　(θ)　between　macroscopic　magnetic　moment　M　and　HTHz　can　be　tuned,　achieving　selective　excitations　of　the　two　modes,　that　is,　θ　=　0　for　q-AFM　mode　and　90°　for　q-FM　mode.　Terahertz　absorption　spectrum　results　indicate　that　as　the　magnetic　field　increases,　the　quasi-ferromagnetic　resonance　(q-FM)　of　TmFeO3　single　crystal　shifts　towards　high　frequencies,　and　quasi-antiferromagnetic　resonance　(q-AFM)　transits　to　q-FM　under　low　critical　magnetic　fields　(2.2–3.6　T).　Through　magnetic　structure　analysis　and　theoretical　fitting,　it　is　confirmed　that　the　magnetic　moment　of　the　single　crystal　undergoes　magnetic　field　induced　spin　reorientation.　This　study　is　helpful　in　better　understanding　of　the　regulation　mechanism　of　the　internal　magnetic　structure　of　rare　earth　ferrite　under　the　combined　action　of　external　magnetic　field　and　temperature　field,　and　also　in　developing　related　spin　electronic　devices.　©　2024　Institute　of　Physics,　Chinese　Academy　of　Sciences.　All　rights　reserved.