Energy-efficient tunability is highly desired for silicon photonic devices. We demonstrate a thermo-optic tunable filter with an ultra-high tuning efficiency based on a suspended photonic crystal nanobeam cavity. Attributed to the ultra-small mode volume and free-standing waveguide structure, a tuning efficiency of 21 nm/mW is achieved over a wide single-resonance tuning range of∼43.9 nm
. The10%–90% switching times are 67.0 μs and 68.8 μs for the rising edge and the falling edge, respectively. The demonstrated energy-efficient tunable device can find applications in reconfigurable photonic integrated circuits.