We present a post-treatment method for directly-spun ultra-thick carbon nanotube fibers (CNTFs) with a high linear density of similar to 6 tex (g km(-1)), which can possibly meet industrial-scale productivity. The treatment consists of consecutive steps of swelling of CNTFs in chlorosulfonic acid, stretching, and coagulation. The swelling of CNTFs makes even distribution of the CNT bundles, which are the subunits of CNTFs, and thereby promotes the removal of the interstitial voids and macroscopic pores during subsequent coagulation. The stretching straightens tortuous CNT bundles, and thereby facilitates dense packing of aligned bundles along the fiber axis that are evenly distributed by swelling. However, highly-tortuous CNT bundles in the high linear-density CNTF leads to incremental improvement of structure and properties. The stretching is only effective at stretching ratio (R-S) > 20%, and both tensile strength and electrical conductivity gradually increase as R-S increases, to 27- and 8.7-times at the maximum R-S = 100%. We believe that our study on the influence of R-S on the structure and properties of high linear-density CNTFs provide a new opportunity for designing the industrial process of the post-treatment to commercialize CNTFs.