Abstract: Aperture fill time effect on phased array radars was analyzed. From the analysis of the time domain and frequency domain, it can be seen that the aperture fill time effect will cause deviation of the beam direction and the attenuation of power. The limit to relative bandwidth was put forward according to the demand from beam direction deviation. The analysis of the frequency spectrum indicated that the aperture fill time would cause power attenuation at the margin of the band width and main lobe broadening after the pulse compressing. Specific constraint was put forward according to the demand of power attenuation. Theoretical analysis indicates that the wideband phased array radar needs stricter constraint to reduce the aperture fill time effect.