In order to detect the polarization properties of the target, a dual-polarized monopulse Cassegrain antenna at W-band is proposed. The antenna is composed of the main-reflector, the sub-reflector, the feed horns, the orthomode transducer, and the sum-differential network. The aperture diameter of the antenna is 137 mm, the feed contains five horns, the orthomode transducer is designed using the tapered waveguide, and the sum-differential network is realized by the planer coupler. The feed horns, orthomode transducer, and sum-differential network are integrated designed to reduce the insertion loss and compress the size of the antenna. The measured results show that the impedance bandwidth of the antenna is about 3.7 GHz, the polarization isolation is greater than 35 dB, the maximal gain of the sum beam is about 37.9 dBi, the sidelobe level is below -15 dB, and the null depth of the differential beam is better than -25 dB. The antenna shows good dual-polarized monopulse performance at W-band.