An organic thin film transistor and a manufacturing method thereof are provided. The organic thin film transistor comprises: a substrate; a gate electrode layer (21) and a source/drain electrode layer (24), formed on the substrate; an organic semiconductor layer (25), formed between source and drain electrodes (24) of the source/drain electrode layer; and an organic insulating layer (23), formed between the gate electrode layer (21) and the organic semiconductor layer (25) and made from an organic polymer material.