As the first thermal stable molecule with a B B bond, the diboryne complex protected by N-heterocyclic carbene ligands (NHC-B B-NHC) has attracted much interest. Researchers point out that pi-backdonation highly stabilizes the B B bond besides a-donation, both of which are induced by NHC ligands. In this work, details of the pi-back-donation are revisited by using DFT calculations. There are two delocalized pi* orbitals in NHC, and the symmetry of one e orbital is highly adaptive to then orbitals in B B bond, whereas the other cannot be involved in the A-back-donation. In staggered configuration, two orthogonal pi orbitals of B B interact with this pi* orbital in each NHC ligand, respectively, to form R-back-donations in both sides. This interaction has proven to be more intensive than pi-conjunction, resulting in the lower energy of the staggered isomer compared with the eclipsed one containing greater pi-conjunction. Moreover, intensity of the pi-back-donation can be enhanced by reducing the energy levels of the matched pi* orbitals in ligands, which gives references for the design of stable diborynes.