A high aperture efficiency W-band Cassegrain dual-reflector monopulse antenna is proposed for helicopter collision avoidance radar applications. Aperture field approach method has been investigated to increase the whole performance of the proposed antenna with unequal phase aperture distribution in W-band. The proposed monopulse antenna integrated with the aperture size of 135 mm and focal length of 40.5 mm was fabricated. A sum-difference network based on the E-plane branch line bridge was designed to test the antenna. Measured results show that the gain of the antenna is 38.6 dBi at 93 GHz, corresponding to the aperture efficiency of 54.7%. In addition, it owns the null-depth better than -24 dB and the side-lobe below -18 dB. Measured results are in good agreement with predicted ones. It is demonstrated that the proposed structure is an attentive candidate of high aperture efficiency W-band monopulse antenna.