This paper presents an ultra-wide bandwidth (UWB) low-frequency radio astronomical cryogenic receiver for the Five-hundred-meter Aperture Spherical radio Telescope (FAST). It covers 6.6:1 bandwidth from 0.5 to 3.3 GHz. The receiver consists of a Quad-Ridged Flared Horn (QRFH), a cryogenic microwave unit, an optical transceiver and a warm microwave and frequency mixing unit. A QRFH with a concentric-loaded dielectric spear is developed: the average return losses are larger than 20 dB; the average ports polarization isolation is 43.87 dB; the average dish efficiency is higher than 65%. Many UWB cryogenic low loss components are developed for the fabrication of a cryogenic microwave unit. The average noise temperature lower than 14.2 K and 22.5 K are achieved as referred to the input ports of cryogenic Dewar and the output of horn, respectively. Compared to other similar advanced UWB receivers, such as Parkes 0.7–4.2 GHz (6:1 bandwidth) receiver and FAST 0.27–1.62 GHz (6:1 bandwidth) receiver, wider relative bandwidth of the proposed receiver is achieved and it is a new attempt to expand the bandwidth of UWB low-frequency receiver.