A compact spaceborne Ku-band wideband fan beam circularly polarized antenna is proposed in this paper.It has a meander-line polarizer (MLP) which located in front of the horn to achieve circular polarized characteristics. By using a semi cylindrical circular polarizer, the size of the antenna is reduced by nearly 36%. Meantime, to adjust the spherical wave to a plane wave within a limited size, a dielectric lens is added at the edge of horn. The design of semi cylindrical polarizer and dielectric lens can jointly adjust the incident beam angle and improve the wide-angle beam coverage performance of the antenna. To validate the design theory, a Ku-band circular polarized antenna is designed and simulated by CST. The antenna has compact structure and wide beam gain is greater than 11.3dBi which the beam width of the elevation plane is more than 90°. The design of semi cylindrical polarizer provides a new idea for the miniaturization and low-cost design of spaceborne antenna.