The high-quality preparation and optimization of semiconductor-based carbon nanotube films is of great significance for carbon nanotube-based electronic devices. The length of semiconducting carbon nanotubes is one of the important factors affecting the thin film. In this paper, high-purity semiconducting carbon nanotube solution was successfully prepared by poly［9-(1-octanoyl)-9H-carbazole-2,7-diyl］ (PCz). Through a cyclic deposition process, the short carbon nanotubes were effectively removed, and the average length of semiconductor carbon nanotubes was improved. On this basis, high-performance carbon nanotube thin film transistors were successfully prepared through standard process. The results show that the thin film transistor prepared by the optimized long carbon nanotube solution presents excellent electrical properties, with an on-off ratio of up to 107 and a mobility of up to 34cm2·V-1·s-1, which is 3 times higher than that of short tubes.