Embodiments of the present disclosure provide preparation methods for a semiconductor layer and a TFT, a TFT and an array substrate. The preparation method for a semiconductor layer includes forming a silicon dioxide film on a substrate; forming sidewalls at two ends of the semiconductor layer to be formed by patterning process; performing amination treatment on the sidewalls so that an aminosiloxane monolayer self-assembly is formed on the surface of the sidewalls; carboxylating a carbon nanotube solution and making the carboxylated carbon nanotube solution on the surface of the substrate with the sidewalls formed to form a carbon nanotube film; removing portions of the carbon nanotube film other than the portion between the sidewalls to form a semiconductor layer.