We report a thermo-optic switching effect with a high extinction ratio of 30 dB by means of filling a fluid into air holes of a solid-core photonic crystal fiber (PCF). Such an effect can perform a turn ON–OFF operation of the transmitted light via a small temperature adjustment of 10 C. The switching function attributes to the absorption of the filled fluid in combination with the interaction between the core mode and the excited “fluid rod” modes, resulting from the thermo-optic effect of the filled fluid.