The invention discloses a flexible organic thin film transistor and a fabrication method thereof. The flexible organic thin film transistor sequentially comprises a flexible substrate, a gate electrode, an insulation layer, a semiconductor layer and a source-drain electrode, wherein the flexible substrate is polyimide, polyethylene terephthalate, polyethylene naphthalate or paper, the gate electrode is one of an indium tin oxide thin film, aluminum-doped zinc oxide, silver or nanometer silver wire thin film, a conductive polymer PEDOT:PSS or PEDOT:PSS and an AgNWs composite thin film, the insulation layer is a cross-linking agent obtained by esterification reaction of poly4-vinyl-phenol and 4,4'-(hexaisopropenylancetylene) phthalic anhydride, the semiconductor layer is C8-BTBT, and the source-drain electrode is gold, or silver, or nanosilver or PEDOT:PSS. The flexible organic thin film transistor has the advantages of high mobility, low working voltage, ideal switch ratio, favorable flexibility and the like, and a solution method is simple in preparation process and good in economic benefit and can be suitable for production on a large scale.