In　this　paper,　we　implemented　diffraction　tomography　(DT)　by　using　a　continuous-wave　(CW)　terahertz　(THz)　source.　An　off-axis　digital　holographic　interferometry　was　built　to　measure　the　complex　amplitude　distributions　of　the　sample　at　various　rotational　angles.　Based　on　the　Rytov　approximation,　the　three　dimensional　(3D)　refractive　index　(RI)　distribution　of　the　polystyrene　(PS)　foam　spheres　was　reconstructed　by　the　filtered　backpropagation　(FBPP)　algorithm.　For　enhancing　the　quality　of　the　reconstructed　RI　tomograms,　the　auto-focusing　algorithm　was　applied　to　correct　the　decentered　error　of　the　sample　when　placed　on　the　rotation　configuration.　This　study　demonstrated　that　THz-DT　can　be　applied　to　reconstruct　the　high-precision　3D　RI　distribution,　which　has　extensive　application　prospects　in　3D　imaging　and　nondestructive　testing.