A highly sensitive optical fiber temperature sensor based on the thermal shift of the liquid–gas interface is proposed. A section of silica capillary tube (SCT) was spliced to a single mode fiber, and a small quantity of liquid polydimethylsiloxane was filled into the innermost part of SCT by using the offset capillarity method. Then, the end of the SCT was sealed by solidified UV curable adhesive. As a result, a liquid Fabry–Perot (FP) cavity, an air FP cavity, and a liquid–gas interface were formed simultaneously in the SCT. The lengths of the two cavities changed synchronously and complementarily along with the thermal shift of the liquid–air interface, which resulted in the opposite wavelength shifts of the corresponding FP interference fringes. The temperature sensitivities are 1.67 and −0.32  nm  /    °  C, respectively. A differential sensitivity of 1.99 nm/°C can be achieved by monitoring the two kinds of fringes synchronously.