Terahertz　waves　of　which　frequency　spans　from　0.1　to　10　THz　bridge　the　gap　between　the　infrared　spectrum　and　microwaves.　Owing　to　the　special　features　of　terahertz　wave,　such　as　penetrability　and　non-ionizing,　terahertz　imaging　technique　is　a　very　significant　and　important　method　for　inspections　and　detections.　Digital　holography　can　reconstruct　the　amplitude　and　phase　distributions　of　a　sample　without　scanning　and　it　already　has　many　successful　applications　in　the　area　of　visible　and　infrared　light.　The　terahertz　in-line　digital　holographic　multi-plane　imaging　system　which　is　presented　in　this　paper　is　the　combination　of　a　continuous-wave　terahertz　source　and　the　in-line　scheme　of　digital　holography.　In　order　to　observe　a　three　dimensional　(3D)　shape　sample　only　a　portion　of　which　appears　in　good　focus,　the　autofocusing　algorithm　is　brought　to　the　data　process.　The　synthetic　aperture　method　is　also　applied　to　provide　the　high　resolution　imaging　effect　in　the　terahertz　waveband.　Both　intrinsic　twin　images　and　defocused　objective　images　confuse　the　quality　of　the　image　in　an　individual　reconstructed　plane.　In　order　to　solve　this　issue,　phase　retrieval　iteration　algorithm　is　used　for　the　reconstruction.　In　addition,　the　reconstructed　amplitude　image　in　each　plane　multiplies　the　mask　of　which　the　threshold　depends　on　the　values　of　the　autofocusing　curve.　A　sample　with　simple　artificial　structure　is　observed　which　verifies　that　the　present　method　is　an　authentic　tool　to　acquire　the　multi-plane　information　of　a　target　in　terahertz　waves.　It　can　expect　a　wide　application　in　terahertz　defect　detecting,　terahertz　medical　inspection　and　other　important　areas　in　the　future.