Reconfigurable cavity-backed slot antennas (CBSAs) using fluid dielectric is proposed in this paper. On one hand, dielectric post, i.e., distilled water, can produce an effective permittivity of the resonant mode, and varying amount of the dielectric can achieve controlled as well as the resonant frequency. On the other hand, the energy of the resonant mode can radiate through the slot on the top wall. Thus, a reconfigurable cavity-backed slot antenna can be realized. Besides, as the resonant mode is only affected by the dielectric post along the direction of the corresponding electric field, three cavity modes with orthogonal electric field, i.e., TE101, TE011, and TM110, can be used to design reconfigurable tri-band slot antenna.