Abstract: This paper presents the ammonia gas sensor of optical fiber dislocation type based on single-walled carbon nanotubes (SWCNTs)-polymer self-assembled composite film. Alone with high-Q resonator, the film has a large number of free carboxyl groups and a large specific surface area, which provides strong interaction between light and the film, as well as high adsorption and selectivity to ammonia. The spectrum from the sensor varies with the effective refractive index affected by the ammonia concentration. In the low concentration range of (10−37)×10⁻⁶, the ratio of the spectral change to the ammonia concentration difference (i.e. the sensitivity) is of 13.25 pm/10⁻⁶, a detection limit is 3.77×10⁻⁶ with good linearity. This work provides an effective method for developing low-concentration and high-selectivity ammonia sensors.