Abstract: A hybrid method combining parallel finite-difference time-domain (PFDTD) and parallel time-domain physical optics (PTDPO) is presented for computing radiation fields of large dual reflector antennas. The radiation fields can not be simulated by singular time-domain methods and the accurate results couldnt be got by singular frequency technologies. Two process groups for parallel computing are firstly set up, one is for PFDTD computing and the other is for PTDPO computing. Then the near magnetic fields of the dual reflectors feed are got by the PFDTD method. Based on these fields, the magnetic fields near or on the sub-reflector are parallel computed with Kirchhoffs surface integral representation (KSIR). The magnetic fields on the main reflector are also obtained by KSIR. The radiation far-fields of the dual reflector are calculated by the PTDPO method at last. The radiation patterns of the Cassegrain dual reflector antenna, whose main-reflector aperture size is 40 wavelengths, show the results near the axis simulated by the PFDTD-PTDPO hybrid method agree well with those by the PFDTD method. The radiation simulation in time domain near the axes of dual reflectors with the same feed and different aperture size is given. The hybrid method has the ability of simulating any large dual reflector antennas with the aperture size of hundreds of wavelengths, even if the reflectors feed is very big or offset.