A highly sensitive microfiber ammonia (NH3) sensor was fabricated by means of flame melting taper in this paper. The sensor is fabricated by splicing a length of 10 mm Polarization Maintaining Fiber (PMF) in the middle of ordinary Single Mode Fibers (SMF), stretching the PMF down to microscale with diameter of 8.33 μm by using optical fiber melt tapering machine. Based on the principle of Mach-Zehnder Interferometer (MZI), this structure utilizes the interaction of PMF core and cladding modes to achieve inter-mode interference. When the NH3 concentration changes in the external environment, the evanescent field in the cone area changes. By detecting the wavelength shift of the transmission spectrum, the concentration of ammonia can be measured by fabricated sensors in this paper. The experimental result shows that the sensor has a quadratic corelation to ammonia concentration in the range of 8 ppm-56 ppm, and the wavelength shift about 5nm in the direction of the long wave. The experimental result also shows that the sensor has a linear corelation to ammonia concentration with sensitivity of 176.08 pm/ppm in the range of 32 ppm-56 ppm. The sensor has the advantages of small size, easy fabrication and high sensitivity making it a good candidate for NH3 sensor in different fields.