The invention discloses an OFET (Organic Field Effect Transistor) ammonia gas sensor containing a functional insulation layer. The OFET ammonia gas sensor comprises a substrate, an insulation layer a, the functional insulation layer, an insulation layer b and an organic semiconductor layer which are sequentially arranged from bottom to top, wherein the organic semiconductor layer is provided with a source electrode and a drain electrode; a gate electrode is arranged between the substrate and the insulation layer a; the insulation layer a and the insulation layer b are formed by polymer insulation materials which do not contain hydroxyl; the functional insulation layer is formed by a polymer insulation material which contains the hydroxyl. According to the OFET ammonia gas sensor disclosed by the invention, a functional insulation layer material containing the hydroxyl is introduced between two insulation layers, so that the gas sensing property of an OFET is increased; -OH contained in the hydroxyl in the functional insulation layer can be subjected to reversion when ammonia gas molecules permeate into an interface of the insulation layers/the organic semiconductor layer, so that hole charges in corresponding number can be induced, parameters of saturation current, a migration rate and the like of devices are enabled to change, and detection on ammonia gas can be realized.