In this study, a high-sensitivity temperature sensor based on the sandwich multimode fiber Mach-Zehnder interferometer (MZI) with the virtual vernier effect is developed, and theoretical analysis and experimental verification are performed. The MZI was made by splicing a graded-index multimode fiber with a length of 20 mm between two pieces of stepped-index multimode fibers with lengths of 1 mm. The sandwich multimode fiber MZI was used as the sensor interferometer to evaluate the vernier effect, and the reference interferometer spectrum was obtained via frequency conversion of the interference spectrum of the fiber MZI through signal processing. The virtual vernier spectrum was obtained by superimposing waveforms of the sensing and reference interferometers. The experimental results show that the temperature sensitivity of the sensor system is 3.884 nm/℃ within the temperature range of 40-100 ℃, which is 37.346 times higher than that of a single-sandwich multimode fiber MZI. Compared with the traditional fiber sensors based on the vernier effect, the proposed sandwich multimode fiber MZI temperature sensor based on the virtual vernier effect has exceptionally high sensitivity, compact size, simple fabrication, low cost, and more reliable results.