A magnetic impurity embedded in a Fermi sea is collectively screened by a cloud of conduction electrons to form a Kondo singlet below a characteristic energy scale TK,the Kondo temperature,through the mechanism of the Kondo effect.We have reinvestigated the Kondo singlet by means of the newly developed natural orbitals renormalization group (NORG) method.We find that,in the framework of natural orbitals formalism,the Kondo screening mechanism becomes transparent and simple,while the intrinsic structure of a Kondo singlet is clearly resolved.For a single impurity Kondo system in whichever case of either finite size or thermodynamic limit,there exists a single active natural orbital that screens the magnetic impurity dominantly.In the perspective of entanglement,the magnetic impurity is entangled dominantly with the active natural orbital,i.e.,the subsystem formed by the active natural orbital and the magnetic impurity basically disentangles from the remaining system.We have also studied the structures of the active natural orbital respectively projected into real space and momentum space.Moreover,the dynamical properties,represented by one-particle Green's functions defined at the active natural orbital,are obtained by the correction vector method.Meanwhile,the well-known Kondo resonance is clearly observed in the spectral function at the active natural orbital.To realize the thermodynamic limit,the Wilson chains with the numerical renormalization group approach are employed.