The invention provides a compact broadband circular polarizer which comprises a square-circle converter, a mode converter, a reflecting back cavity and a feed source. The square-circle converter is connected to the upper end of the mode converter through a flange plate, and the reflecting back cavity and the feed source are connected and are connected to the lower end of the mode converter through the flange plate. The compact broadband circular polarizer is simple in structure, small in size, capable of covering wide frequency bands, capable of using the same circular polarizer for meeting the demands of mobile communications, satellite communications, short distance communications, broadband access communications and digital TV receiving with different standards in different areas, capable of meeting the demand for system miniaturization, and wide in application prospect.