In virtue of wave-front coding theory, a computer simulation analysis of the depth of field extension of negative refraction flat lens is carried out. The depth of field extension can be achieved by 6.7 times in the slab, when its index is -1, through computer simulation. When the refractive index of the tablet is slightly offset, taking -0.99 for example, 6.9 times of the depth of field extension can be achieved after the introduction of the calibration lens.