Abstract: The wake field is stimulated when a slice electron bunch transfers in the homoge neous plasma,and it transfers forward after the bunch.The wake field can be used to be the wiggler for SASE-FEL.The wiggler mechanism is analyzed and its space period,field intensity and the gain length are calculated.Based on SASE theory,this kind of wiggler is compared with APS wiggler.The result shows that the wake field wiggler is superior to the normal wiggler used currently.