A broadband high-gain Right-Hand-Circularly-Polarized(RHCP) microstrip-horn combined antenna is proposed. Firstly, a broadband circularly-polarized microstrip antenna with double-layer is designed, which has a property that the overlapped bandwidth of Voltage Standing Wave Ratio(VSWR)≤2 and Axial Ratio(AR)≤4 dB is more than 24%, and the RHCP gain is about 9 dBi. Then, the gain is enhanced significantly by adding the pyramidal horn structure, while the other characteristics of the microstrip antenna are slightly changed. The parametric studies are carried out to investigate the effects of primary geometric sizes on the antenna’s performance.