High intensity radiated field construction system is a key equipment for electromagnetic irradiation effect test of various weapon systems. It can excite high intensity and evenly distributed electromagnetic field in a certain distance from the antenna. In this paper, an X-band offset Cassegrain multimode reflector antenna is designed for this system. Reflector antennas are used to obtain high gain so that the field strength in the desired region is as large as possible. The flat top narrow beam is realized by using the theory of multimode reflector, which makes the field in the desired area tend to be evenly distributed, while the field outside the area decreases rapidly. The measured results show that the gain of the proposed antenna is greater than 29.8 dB, and the 3 dB beamwidth is not less than 4.6°. In this range, the amplitude fluctuation of the pattern is less than 2 dB, and the flat top characteristic is obvious. In addition, the dual-bias reflector antenna has the advantages of small feed occlusion, low feeder loss and easy folding, which can be well applied to electromagnetic environment simulation test equipment.