In microwave energy transmission and wireless communications system, the antenna needs to be low-profiled, and easy to be integrated with planar circuits. A novel single fed cavity backed circularly polarized ring slot antenna with harmonic suppression function is proposed. The antenna operates at 5.8 GHz and adopts a novel guided wave structure, Substrate Integrated Waveguide(SIW), to suppress surface waves and increase antenna gain. The substrate integrated waveguide is integrated on the substrate through two rows of metal via, and the short circuit annular gap is etched on the substrate to radiate electromagnetic waves. Antenna fed by 50 Ω microstrip line to SIW transition is adopted, and a parallel branch in the feeding line is introduced to realize suppressing harmonics based on the substrate integrated waveguide structure. Through fabricating and testing, the axial ratio of the antenna is 0.67 dB, the gain is 6.9 dBi, the impedance matching is good, and the front to back ratio is 22 dB. The simulated results are in good agreement with the measured results.