Three-dimensional　(3D)　refractive　index　(RI)　distribution　is　important　to　reveal　the　object＇s　inner　structure.　We　implemented　terahertz　(THz)　diffraction　tomography　with　a　continuous-wave　single-frequency　THz　source　for　measuring　3D　RI　maps.　The　off-axis　holographic　interference　configuration　was　employed　to　obtain　the　quantitative　scattered　field　of　the　object　under　each　rotation　angle.　The　3D　reconstruction　algorithm　adopted　the　filtered　backpropagation　method,　which　can　theoretically　calculate　the　exact　scattering　potential　from　the　measured　scattered　field.　Based　on　the　Rytov　approximation,　the　3D　RI　distribution　of　polystyrene　foam　spheres　was　achieved　with　high　fidelity,　which　verified　the　feasibility　of　the　proposed　method.