Multi-reflector based collimation antennas are widely used in microwave remote sensing and millimeter wave imaging. For design and analysis of these antenna systems, many methods have been introduced of which the Diffracted Gaussian beam approach (DGBA) is among the most popular. For electrically large reflector antennas, the computation of reflection and diffraction for gigantic number of elementary Gaussian beams demands huge computational resources. To simulate a reflector antenna in hundreds to thousands of wavelength, it may require independent Gaussian beams ranging from tens of millions to a billion, which is extremely time-consuming on conventional computers. To this end, the parallel computer provides one of ultimate solutions since reflection and diffraction of each Gaussian beam can be simulated separately on different CPUs simultaneously. In this paper we present the DGBA simulation of a reflector antenna system on high performance parallel computing platforms, based on Message Passing Interface (MPI).