A fiber ammonia gas sensor with high sensitivity is realized by combining single taper-based microfiber modal interferometer with graphene film. It uses grapheme’s specific absorption property to ammonia and high sensitivity sensing characteristics of microfiber structure, and the detection of weak changes of ammonia concentration is realized by detecting the shift of interference spectrum. Comparison and analysis are carried out among different groups of sensors. Results show that when the fiber diameter is 3.4 μm, the maximum detection sensitivity is 10.8 pm/ppm, which is almost doubled compared to the results of other reported fiber structures. The proposed sensor has the advantages of simple structure, easy implementation and high sensitivity, and it has potential application prospect in the fields of harmful gases concentration alarm and human health detection etc.