Hollow cylindrical carbon nanotube (CNT) sock was synthesized via floating catalyst chemical vapor deposition. The synthesis process was carried out in a horizontal furnace by injecting feedstock consisting of ethanol, ferrocene, thiophene, and deionized water into the alumina tube with argon as carrier gas. In order to investigate the formation mechanism of the hollow cylindrical CNT sock, the influence of the injection rate and C/Fe molar ratio of feedstock on the nucleation and growth of CNTs was discussed. The products were characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and thermo-gravimetric analysis. The results showed that hollow cylindrical CNT sock was obtained when those two parameters were 30 ml h(-1) and 523, respectively. A deposition-float-assembly (D-F-A) formation mechanism was proposed for the formation of CNTs sock. Firstly, the as-prepared CNTs were deposited on the inner surface of the alumina tube. Secondly, with the help of buoyant effect for Fe catalyst nanoparticles, CNTs floated in the alumina tube with longer length. The long CNTs were assembled into interconnected CNT membranes. The alumina tube was blocked by the accumulation of CNT membranes to form a self-closing system. The CNT sock was obtained by drawing out the block stably. The D-F-A formation mechanism is of great significance in the fabrication of the hollow cylindrical CNT sock.