Terahertz　digital　holography　is　a　combination　of　terahertz　imaging　and　digital　holography.　During　reconstruction,　the　key　point　is　to　find　accurately　the　propagation　distance　from　which　the　distribution　of　focused　samples　can　be　reconstructed.　In　this　paper,　we　use　a　continuous-wave　terahertz　in-line　digital　holographic　imaging　system　to　record　holograms.　Moreover,　the　autofocusing　algorithms　through　which　the　reconstructed　distance　can　be　calculated　are　applied　to　the　reconstruction.　The　in-line　schematic　is　beneficial　to　the　terahertz　wave　imaging,　which,　however,　inevitably　produces　the　object＇s　twin　image.　In　the　refocusing　process,　both　the　reconstructed　image　with　low　signal-to-noise　ratio　and　contrast　and　the　twin　image　induce　the　formation　of　false　peaks　corresponding　to　improper　distances　on　the　autofocusing　curves.　To　restrain　the　disturbance　factors　and　improve　the　accuracy　of　the　judgment,　a　phase　retrieval　method　is　implemented　in　the　reconstruction.　The　results　demonstrate　the　feasibility　of　the　autofocusing　method　with　phase　retrieval　in　terahertz　in-line　digital　holographic　imaging　system.　The　proposed　method　provides　an　automated　and　efficient　evaluation　which　helps　to　obtain　the　optimized　propagation　distance.　(C)　2015　Elsevier　B.V.　All　rights　reserved.