In recent years, as a new type of guided wave structure of microwave system, SIW cavity is popular in the industry, SIW cavity miniaturization becomes a research hotspot too. A new type of half mode substrate integrated waveguide(HMSIW)right-hand circularly polarized slot antenna is designed in this paper. It is a half-arc SIW cavity with a vertical gap of the same width and different length etched at the top of it. As an excitation source, a single stage embedded microstrip whose input impedance is 50Ω is connected to the bottom of the substrate. The simulated results show that to right-hand half mode circularly polarized, the center frequency is 9.49GHz, VSWR is 1.08 at this point and the maximum gain can reach 4.63dB. By the physical production, processing and testing of the antenna, it shows that compared with the traditional circularly polarized antenna the size of this kind of antenna can decrease by nearly 50%. It realizes the miniaturization of the circularly polarized antenna, and performance and the simulated results are basically identical.