Prestack time migration is one of effective migration method for a complex structure imaging,which can adapt to the imaging of steep dip angle and the large changes in vertical and horizontal velocity. The offset aperture and migration velocity are critical to imaging. If migration aperture is over small, the offset profile will loss the common phase axis of steep dip. If migration aperture is over big, it will reduce the quality of migration data. Though the concept of migration aperture is completely different from that of field acquisition aperture,the migration aperture is constrained by the acquisition aperture. If acquisition aperture is not enough many, it will not precisely image the data of steep-dip and middle-deep formation. So the suitable design of field acquisition aperture is crucial to the final imaging quality. To the deepest target area of a survey, the migration aperture comparatively depends on the acquisition aperture and the migration aperture only needs a slightly larger twice of the largest offset. If acquisition aperture can not satisfy the imaging demand of the target area, no matter how long the migration aperture in data processing is added to, the imaging quality of steep dip angle in target area can not be improved also.