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In this study, a W-band reflectarray antenna is designed with the characteristics of low phase sensitivity and wideband. Low phase sensitivity is achieved by subdividing the unit cell variation method into five steps. While designing a unit cell with low phase sensitivity, phase distortion may occur within the target bandwidth with varying incident angles of illumination, resulting in a limited bandwidth. The origin of large-phase distortion is analyzed based on the input impedance of multiresonant elements, considering the incident angle. This study suggests a selection strategy of the variation steps in the reflectarray by using the relative phase requirement based on the structural dimensions of the proposed antenna. An offset feed reflectarray with 12λ₀ aperture diameter at 90 GHz center frequency is designed using the proposed method. The measured gain is 28.5 dBi at 90 GHz, and the maximum efficiency is 55.3% at 82 GHz. The measured 1-dB gain bandwidth is approximately 28.8% in the W-band of 81-107 GHz.