In this work, we designed a series of superalkali‐doped Si12C12 nanocage M3O@Si12C12 (M = Li, Na, K) with donor–acceptor framework. Density functional theory calculations demonstrated that the HOMO–LUMO gap of the complexes conspicuously narrowed with increase of atomic number of the alkali metal, the value decreased from 5.452 eV of pure Si12C12 nanocage to 3.750, 2.984, and 2.634 eV of Li3O@Si12C12, Na3O@Si12C12, and K3O@Si12C12, respectively. This finding shows that the pristine Si12C12 cluster could be transformed to n‐type semiconductor by introduction of the superalkali M3O. We also showed that the superalkali doping remarkably enhanced the first hyperpolarizability of Si12C12. Among the studied systems, K3O@Si12C12 not only has the narrowest gap but also has the strongest nonlinear optical (NLO) properties, its first hyperpolarizability reached as high as 21695 a.u. The striking results presented in this work will be beneficial for potential applications of the Si12C12‐based nanostructure in the electronic nanodevices and high‐performance NLO materials. © 2017 Wiley Periodicals, Inc. This work designed a series of M3O@Si12C12 (M = Li, Na, K) nanoclusters with remarkable nonlinear optical property. The first hyperpolarizability of the systems sharply increases with rising atomic number of alkali atom in M3O moiety.