The receiving characteristics of the 90 degrees offset parabolic reflector antenna with multi-beam at millimeter-wave band are analyzed. A tentative analysis is made by using the plane receiving array instead of the traditional curved surface receiving array. The diffraction speckle on the receiving plane of the incident beam with various angles is calculated and analyzed by adopting the combination method of Ray-tracing and Stratton-Chu vector diffraction integral. The numerical results indicate that it is available by using the plane receiving array instead of the curved surface receiving array to the beam in some incident angles in the 90 degrees offset parabolic reflector antenna at millimeter-wave band.