Soon after the discovery of superconductivity in layered Fe-Ch systems, the intense search of related Fe-free materials has been begun. It was found that these systems such as K x Ni 2 (S,Se) 2 are cation-deficient, and the superconducting state should be sensitive to the presence of K and Ni vacancies. Herein, using the first principles FLAPW-GGA calculations, we have studied the electronic structure and Fermi surface (FS) topology for cation-deficient K 1− x Ni 2− y Se 2 . We have found that the presence of potassium vacancies (for K 0.8 Ni 2 Se 2 ) as well as the joint effect of K and Ni vacancies (for K 0.8 Ni 1.6 Se 2 ) change essentially the topology of their FSs and the character of electronic bands. Unlike 2D-like FS for stoichiometric KNi 2 Se 2 , the FSs for cation-deficient compositions demonstrate much more complex topology of mixed 2D-3D type.