The invention belongs to the technical field of a sensor, and discloses an organic field effect transistor-based ammonia gas sensor and a preparation method therefor, and aims to solve the problem ofhigh cost of the existing organic field effect transistor-based ammonia gas sensor due to a large amount of consumed manpower and materials. The organic field effect transistor-based ammonia gas sensor comprises a substrate layer, a gate electrode, a dielectric layer and an organic semiconductor layer which are arranged from the bottom up in sequence; the upper end of the organic semiconductor layer is connected with a source electrode and a drain electrode; the ammonia gas sensor is characterized in that the dielectric layer adopts the material of egg white protein; and the organic semiconductor layer is prepared from a soluble organic semiconductor material with 5-15mass% of egg white protein.