This paper presents the design of a low-profile, polarization-reconfigurable broadband substrate integrated waveguide (SIW) antenna based on the SIW structure, with a thickness of approximately 2.5%λ. By introducing a circular slot on the SIW cavity, two radiation resonant frequencies are excited. The adjustment of the via positions within the SIW cavity allows for the control of these two resonant frequencies, thereby extending the operating bandwidth of the low-profile antenna. Additionally, by controlling the RF diodes mounted on the surface of the SIW antenna, the two radiation resonant frequencies can be manipulated, enabling the reconfiguration of the antenna′s left-hand and right-hand circular polarizations. Simulation and experimental results demonstrate that the low-profile SIW antenna, with a thickness of only 2.5%λ, achieves a relative operating bandwidth of 6.4%. Within the frequency range of 5~5.3 GHz, the antenna gain exceeds 5 dBi. Furthermore, the antenna can achieve reconfiguration between left-hand and right-hand circular polarizations within the frequency range of 5.17±0.035 GHz, with a polarization isolation better than 15 dB.