The crystal structure，electronic structure and magnetism of the double perovskite Sr2FeWO6 and SrKFeWO6 have been investigated under the framework of density functional theory (DFT) with the generalized gradient approximation taking into account the on-site Coulomb energy (GGA+U) using the projected augmented wave (PAW) method. Structure relaxation results show that K doping of Sr2FeWO6 stabilizes FeO6，WO6 octahedra and makes the Fe—O—W angles close to 180°，indicating the enhancement of superexchange interaction. From the electronic structure calculation，it was found that the contribution to the total density of states (DOS) from K itself is small. Due to the K doping，the valence and magnetic moment of B-site cation Fe are enhanced and the hybridization between Fe and O becomes stronger，as well as the band gap is enlarged. Nevertheless，it does not cause considerable change in B’-site cation W. The process of transfer of electrons is dominated by Fe-Fe in SrKFeWO6 compared with Fe-W charge transfer in Sr2FeWO6 before doping.