It is found that two walls of fiber micro-cavity fabricated by femtosecond laser micromachining are not perpendicular to the fiber axis. Interference spectrum of the unparallel wall fiber micro-cavity Mach-Zehnder interferometer (MZI) shows abnormal characteristics, such as optical path difference decreasing linearly with wavelength increasing and the total loss decreasing with wavelength increasing. In this regard, we propose an unparalleled wall fiber micro-cavity MZI model and establish analytical theory. By using new models and theories, the new micro-cavity interferometer characteristics are studied, including that the effects of corner and depth on spectral peak wavelength are numerically analysed and transmission loss, absorption loss, insertion loss, infrared absorption loss of material as well as how they affect the interference fringe contrast are theoretically studied. Theoretical analyses and experimental results are in agreement with each other. For fluid sensing, a high-quality unparallel wall fiber micro-cavity MZI is fabricated. The interference fringe contrast of the fiber micro-cavity reaches up to 35 dB in water. Experimental results show that the sensor exhibits an ultrahigh RI sensitivities, as high as——12937.31 nm/RIU in aqueous solution of sucrose.