We present the design, fabrication, and characterization of a suspended slotted photonic crystal (SSPhC) cavity sensor based on the silicon-on-insulator platform. The sensing performance can be dramatically enhanced by the optimized SSPhC cavity as most of the light energy can be distributed in the low index region (∼57%). By measuring the spectrum response of the cavity sensor immersed in NaCl solutions with different mass concentrations, an ultra-high sensitivity around 656 nm/RIU has been experimentally demonstrated. Furthermore, the whole size of the cavity sensor (including the grating couplers) is 320 × 40 µm2, making the high-sensitivity device attractive for the realization of large-scale multi-channel on-chip sensors.