The Vernier effect magnifies optical sensitivity by the superposition of two spectra with slightly shifted frequencies from a sensing interferometer (SIM) and a reference interferometer (RIM). In this study, we demonstrate that the Vernier effect can be obtained through a single interferometer, which detects the changed signal and provides an artificial reference spectrum (ARS) to be superposed with the changed signal spectrum. The ARS extracted by spatial frequency down-conversion of one sensing spectrum in the signal processing is not affected by environmental changes and can be detuned at an arbitrarily small amount with the measured signal spectrum. This approach is simpler and accurate and provides ultrahigh sensitivity. To validate the principle, a Mach-Zehnder (MZ) interferometer based on a dual-mode microfiber was designed for sensing the refractive index (RI) change magnification, and a high sensitivity of 71354.58 nm/refractive index unit (RIU) was obtained with good linearity.