Microfiber Mach-Zehnder interferometer (MMZI) is a widely used optical device. To solve the problems of large difference between the theoretical and experimental spectrum, low calculation efficiency, poor universality in spectrum calculation of MMZI, a fast and accurate calculation method for predicting the interference spectrum of MMZI used in liquid is proposed. In calculations, phase accumulated in transition region of the taper is considered to reach a good agreement between theoretical and experimental spectrum. To further improve the computational efficiency, dependence of the phase accumulation on wavelength is developed by linear fitting, and a non-linear fitting of difference of effective refractive index (∆neff) on fiber diameter, wavelength, temperature and salinity of the surrounding is obtained by the data from commercial software. In addition, a visual interface for displaying the predicted spectrum is established, by which one accurate spectral calculation can be realized within only two or three seconds. Finally, effects of LP03 mode and shape of transition region on transmission spectrum are discussed and the accuracy of this method is also verified by sensitivity. The calculation method demonstrated here provides an accurate and fast spectral prediction method for MMZI, and will afford important reference for sensor design and optimization.