Iron selenide KxFe2 ySe2,as a new superconducting material with Fe-vacancy ordering 1,2,3,has been attracting great interest because its ground state is insulating and the angle-resolved photoemission spectra 2 and first-principles electronic structures 3 show that there are only electron pockets 1 in the Fermi surface,which break the dominant belief that the Fermi surface nesting of the hole and electron pockets is very important for antiferromagnetic order and superconducting pairing.