The invention discloses a preparation method of an OTFT (organic thin-film transistor)-based sulfur dioxide gas sensor. The preparation method comprises the following steps: completely cleaning a substrate and then drying; preparing a gate electrode on the surface of the substrate to form a pattern of the gate electrode; preparing a dielectric layer on a base plate plated with the gate electrode; performing polar solvent treatment on the formed dielectric layer; preparing an organic semi-conductor layer on the formed gate electrode as well as on the base plate covered with the dielectric layer subjected to polar solvent treatment; then preparing a source electrode and a drain electrode to form a source electrode pattern and a drain electrode pattern, wherein the response ratio of gas is improved remarkably, and the lower limit of detected concentration is relatively low. Compared with a monocrystal transistor, an organic thin-film transistor is prepared more easily and is lower in cost; the organic thin-film transistor based on interface modification has a relatively high response speed and can realize rapid gas detection; the manufacturing cost is lowered, and the preparation method is relatively suitable for large scale and industrialized production.