This article deals with the phase transform system in detail. In short, the transform properties of field are discussed with the phase conception of field. A phase transform system can make the phase distribution of an incident field turned into the desirable phase distribution of the emergent field. Our analysis starts with the thin lens as a phase transform system, then derive the phase transform field of the thick lens, and at last put forward the Fresnel phase transform system, which can change an incident spherical wave into an perfect emergent spherical one.