The wavefront coding extension of the depth of field (DOE) for an imaging system is achieved by inserting a third order phase plate into the pupil plane of a spatially incoherent imaging system. The encoded image can be digitally restored to produce a final image with improved depth-dependent details. The characteristics of the system are studied, and the modulation transfer function (MTF) varying with defocus and modulation coefficient,, of the third order phase mask is analyzed, the results about the effects of value on the extension of DOE and the bandwidth are obtained. The results are conducible to the design of wavefront coding optical system and the choice of an appropriate velue.